Here is an old article discussing the topic

https://melmagazine.com/en-us/story/why-so-many-guys-dont-wo...

It really depends on what you mean by huge. But I have to emphatically disagree with your statement. It is simply not humanly possible to get as big as a modern body builder without steroids. They are not just a shortcut they allow people to get much bigger than otherwise is possible.

I think the difference between dumbbell and barbell is just preference often based on the time you've put into each. My dumbbell press is pathetic due to a lack of training.

I think lifting for the looks is part of it. I've also read that most women prefer guys butts to other parts so it's probably a little silly that most guys do more upper body to get the girls.

When I was last going regularly to the gym I knew loads of other guys that worked out. It was near universal that leg day sucked. I was exhausted after doing squats and deadlifts. I'd recover quickly from most upper body stuff. Leg day was still fun but there was a mental battle to put in the effort required. I remember once not being able to drive home after week 6 of the training program Smolov JR with squatting. I pulled over and had to take an hour to recover. I've not had anything close with upper body lifts.

Everyone is different I guess and we are talking about preference.

>It was my understanding that the reason most guys avoid leg day is because they're not "show" muscles.

I think you are right. Many also may avoid legs because the recovery period can be more painful. Its easy to avoid further stressing certain sore muscle groups (arms, chest, abs, etc) post workout, but legs, not so much.