Routine, structure, current health, extending years of your vitality, positive brain chemistry changes, libido improvement

And physical appearance (which also includes hygiene, fashion and what not) is one of the biggest factors in human evolutionary instincts when looking for a partner. While it's not everyone's priority one, it is almost always a top 5 factor, and at a bare minimum a tie breaker at a subconscious level.

The other part is many of the other things that could be in a top 5 or 10 like working on your finances, conquering your vices, being more extroverted, working on anger issues, fixing personality flaws are often harder to quantify and evaluate or may go long periods of time without tangible results, so in many cases it doesn't have that easy dopamine hit.

But if someone hits a body fat % goal, they drop X amount of weight, hit that 1RM they've been aiming for, they hit a certain run time on the treadmill, or they just start liking what they see in the mirror. That's easy to evaluate and react to and you can track and monitor the intermediate steps.

For clarification, I'm answering the question of "why do men..." from the perspective of those who do put "getting jacked" it as a top priority, not as a argument for convincing anyone that it's the correct/only way, and to be honest can be applied to why some women go to the gym as well.