Penny and Zack made up one of the most infamous couples in The Big Bang Theory. While it was certainly a questionable dating choice, the two characters had a long history together. Despite their on-again, off-again status, the relationship really seemed doomed from the start.
As the show went on and Penny's own narrative arc continued to evolve, it became more and more apparent that she and Leonard were perhaps the better couple, as Zack continued to fade into the background. Of course, both parties respectively had their own happy ending but it's easy to think about what could have happened if the pair had given things one last go.
His Intelligence
There are a lot of differences between the two main love interests in Penny's life. One of the things that Penny came to appreciate about Leonard was his intelligence. After having been with Leonard for a while, once Penny went back to Zack, she found his lack of intellect tiresome.
Zack was never the sharpest tool in the shed as it was and this would have impacted the couple down the line. Once Penny began to surround herself with people who genuinely valued intelligence, it became clear that her own potential was being squandered as on multiple occasions she was the smartest in the room.
His Arrogance
While Zack might have been harmless in his ignorance, his arrogance often shone through. At times he could be very sweet and caring and one of the more beloved secondary characters, but he also became very vain and obsessed with his looks. This resulted in becoming a bit of a ladies' man, which Penny didn't totally approve of.
While Zack was charming and handsome, eventually it became clear that hr may have loved himself more than the people he was dating. Ultimately, a relationship between Penny and Zack really couldn't have worked out due to these factors.
Penny's Immaturity
When fans first meet Penny she is very much in over her head. Her immaturity and lack of plans for her future meant that she was a step behind the rest of the group. She was making ends meet at The Cheesecake Factory, but outside of this, she was constantly eschewing her adult responsibilities.
It's not as if she wasn't ambitious, with her dreams of becoming an actress. However, Zack would pull her back further; her own lack of maturity at the time would allow his own arrogance and lack of motivation to halt any progress.
Money Problems
Zack was always pretty wel- off considering his family owned a lucrative menu designing company. This very much differed from Penny's own financial situation, as she continued to find herself in debt.
She wished to become an actress but couldn't support herself enough to really make the dream possible. The duo came from two very different backgrounds and while Leonard was more supportive of Penny, Zack didn't quite understand her financial situation.
Penny's Acting Ambitions
For Penny, The Cheesecake Factory was just a means to an end, with her main goal to become successful in Hollywood.
A successful person in the industry often needs to make personal sacrifices. The fact is, if Penny's career began to take off, Zack would have been left behind, unable to keep up with the ambitions of the talented young actress. This is a factor that could have sabotaged any of her relationships, though.
An Unstable Future
Marriage and kids were things Penny was always unsure about, while Zack was unlikely to have thought that far ahead. The future of the couple would have always remained unsteady. Even moving in together might have been too much for the pair.
Zack would later show that he really could be committed to someone, but it took him a long time to get there. It's possible that he and Penny would have remained in the status quo, never driving themselves to move forward in their relationship and think about next steps.
Their Failed Marriage
Of course, the pair has a failed marriage already behind them. On a trip to Las Vegas, Penny and Zack actually got married, although they believed it to be fake. They were shocked to learn that by law, they were husband and wife.
That definitely didn't bode well for the couple, who both thought their own wedding was a joke, and then did everything they could to get a divorce. It would have been difficult to see them working down the line again with so much baggage to take care of.
On-Again, Off-Again
There have been a lot of terrible and great dates throughout the show but none could have perhaps have gone worse as Zack and Penny's, which would see the latter so fed up that she ended up sleeping with Leonard on the same day.
The fact that Penny and Zack couldn't just commit to one another meant that something was off. Granted, Leonard and Penny had a similar situation to deal with, but Zack provided an even more unstable coupling in comparison to Penny's other relationships.
Penny's Evolution
Throughout the show, Penny would continue to evolve. She of course got the happy ending that she deserved and throughout The Big Bang Theory would step into a new line of work that made her incredibly wealthy. She made lifelong friends, began a family, and grew as a person tremendously.
For the most part, Zack didn't evolve much at all and perhaps isn't capable of that kind of arc. Sure he's a little different when audiences meet him again towards the end of the show, but not different enough to have been able to keep up with Penny's own self-improvements.
The Last Resort
The fact is, Zack was often a bit of a last resort. Penny once dated him just so she wasn't alone on New Years', which isn't exactly a great reason to get back with an ex. Meanwhile, there was something special between Penny and Leonard that she was intrigued by.
For Penny and Zack, she knew exactly what their relationship was. It was low effort but also meant nothing and could be revisited out of necessity if needed. That's not a great foundation to build a full life together, which may indicate why they never worked.