That was a different world with different economics. Disney had (and still does in Orlando) unlimited land, and land allocating works differently when you're budgeting for assumed attendance in the 2020s versus 50 years ago.
@Mad Dog points out some other factors.
I.E. If Disney was to build a 5th park nowadays, they'd immediately need to budget it at 12m+ attendance annually after 5 years and the requisite buildout for that and what percentage of guests need new rooms etc.
Universal is allocating land for a resort that may need to handle guests for 2 dry parks with 10m attendance each after 5-7 years annually of operation. And you need to budget 13,000-15,000 hotel rooms if that comes to pass.