The shopping has already started, the stores are full. The Halloween cash is flowing freely. The costume and entertaining planning has started for some who get creative. Tik Tok and IG are full of Halloween DIY decorations. Home parties can start by Oct 1 since there is competition for dates so invites must get out soon. Halloween involves lots more planning and commitment of time for many more than Christmas does so the work starts early.
Christmas for us is two days, we don't go to any parties. We might go see a light display or like this year we have a Disney trip in December so we'll see some decor. Other than gift shopping which is more about the gift than the holiday we don't begin preparing until the day we put up our decorations. I decorate a lot but it's pretty much the same each year so no preplanning.
Halloween can involve lots of preplanning and that likely changes for families as their kids get older. I have no issue with there being Halloween stuff in the stores now.
NOW if what you mean Disney World specifically because there are almost three months of parties - it is purely a business move for them. People love those parties and were always asking for them to be in summer which is only time some families can go. It's a slower time anyway so exactly why Disney is okay with doing them. The daytime decor is not that much if you aren't into Halloween it isn't a big deal. Every extra Disney has added over the years is to get guests to come, and spend. Halloween is highly successful for them.
We used to go every October and the parks were great - only thing happening was a big golf tournament. They added the Halloween Party and October totally changed, now not the slow time it was.