Because the whole point of EMH is to get people to stay onsite. Getting "heads in beds" is the number one goal of any hotel. An empty hotel room is money that can never be recouped.
They already do. Swan and Dolphin, the Lake Buena Vista hotels, and Shades of Greed...er, Green... pay for the privilege for their guests to participate in EMH. I'm sure it's not on the scale of $300 per guest, but they pay... oh, yes, they pay...
Because the benefit and exclusivity of EMH's are a perk of staying on-site. Disney hotels can do as they wish in their own parks, and of course it encourages visitors to stay on-site (spending more money) and to get into the parks earlier (to spend more money), which is exactly what Disney wants.
Disney doesn't get the money Four Seasons guests are paying to stay at Four Seasons. The money that Disney resort guests pay for expensive lodging is what Disney wants. Almost all deluxe Disney resorts cost over $300/night year-round, and the average non-discounted cost for a no-frills standard room at a deluxe resort is between $400 & $500/night. The most popular resorts & best rooms go much higher, of course. Consider that typical guests in these rooms stay several nights, that they are already not taking advantage of every EMH offered, and that many resort guests never even go to EMH.
So, if you're getting thousands of dollars from a family who typically attends one or two EMH per trip at most, why would you cheapen the goods & let someone staying offsite pay $400 to walk in? Even a family staying a week at the cheapest Disney value resort or Disney campgrounds is paying more than that to Disney.
If people could do that, there would be less reason to stay on site, as you could save money offsite & then pay $400 for EMH, which is much cheaper than paying thousands for your entire stay at Disney. And if you were already spending thousands at somewhere like 4 Seasons or Waldorf, then you'd get the true non-Disney luxury experience and the Disney perks (which is what you'd like, I gather- well, wouldn't we all?) This would further crowd EMH and it would not help Disney financially, as EMH are a selling point for their charming yet overpriced resorts.
I went to Four Seasons with my parents for a doctors meeting loved it so much and all the rest is history. My dad treated some staff from Cirque got free tix and got me hooked.