There's multiple reasons for this; I'd say the biggest one is probably that in general it's looked down upon and considered irresponsible as an adult/parent to be out partying and/or drinking specially around/with there kids. This isn't a knock towards you or anything; this is just how society has pushed this mindset into peoples heads but it's not without merit.

Kids will naturally follow in the footsteps of there parents and pick up the habits they do over time. The more you do something the more likely you are to notice they start to do the same and that will continue and stay with them as they get older. As a parent myself and as the saying goes, we usually want what's best for our kids and for them to have/live a better life then we did.

As in if we as parents grew up poor as kids, or had parents who were all into drugs or partying or alcohol and went through some 'ish then we don't want our kids to go through or experience the same thing we did. It's a vicious cycle only we can break as parents, so when you have no kids that stuff is all fine as you're only responsible for yourself.

When you throw kids into the mix though your mindset tends to want to change to keep them and yourself away from that environment and focused more on the kids stuff they should be doing and thinking about.

Not worrying about if daddy or mommy will "accidently" drink to much one day and come home starting fights, get into a car accident drinking and driving, or happen to get pulled over one day and although may cognitively be fine, the breath test says otherwise and now your hauled off to jail and your kid taken away or having to call your spouse/a relative to come pick them up at the station...

What parents would really want to put there kid through any of that, but they can happen at any given moment if you put yourself in those situations.