Then you have children and you have to shift out of the child role and take the grownup role. At Halloween, it's now on you to provide your child's costume, make sure they're safe, provide treats and chaperone parties. Christmas is even worse with the financial burden of buying presents. You no longer get to simply consume Halloween and Christmas. You are now part of the production of them.
But if you never have kids and don't take on the responsibility of creating fun for others, you can keep having fun yourself. This has been my strategy. My face still lights up when I see Christmas decorations because I know FUN is coming. I won't be baking hundreds of cookies or running up my credit card on toys and games or weighing the moral consequences of perpetuating the Santa Claus lie. I'll just decorate my apartment exactly the way I want to, buy presents for exactly three family members (from my family of origin), go to as many Christmas parties as I'm invited to and soak in the season wherever I go. I'll have my own Christmas party for grown ups where we'll stay up as late as we want and not worry about paying a babysitter.
On Halloween and Christmas (and other such occasions) I don't have to worry about unrealistic expectations or feeling like I've disappointed my kids by not doing everything well. People say that Christmas is for kids and it's just not the same without them, and that's true. There is magic in experiencing such things through the eyes of a child. You can relive a bit of your childhood when you steward your offspring through such holidays. Or you can relive a bit of your childhood all on your own, by continuing to occupy the role of the wide-eyed consumer, delighting in festivities that someone else provided for you, and all you have to do is be there.