To be honest, I think that's perhaps a bit unfair to them.
Disney has done vast amounts in Orlando (and worldwide) to help with their environmental impact, including ensuring that a certain percentage of their landholdings will never be built on, reducing their water use substantially and protecting the hydrological environment around their resorts, ensuring that they grow as much of the produce as they can for the parks and resorts on-site using environmentally sustainable methods (they actually have a full section in Epcot devoted to the research of new methods and technologies for food production!), reducing power use and sourcing more of their electricity from renewables (Mickey-shaped solar panels!), using bio-fuel in their fleets of busses and other vehicles, and more.
They also take part very actively in a number of environmental and wildlife conservation programmes, and fund many others. Some more information can be found here:
https://thewaltdisneycompany.com/environmental-sustainability/
I'm personally a Disney fan and have visited many times. I've also been lucky enough to see "behind the scenes" on many occasions, and to get an idea of what it actually takes to run the place. It's nothing short of incredible, to be honest, and operating at the scale they do costs insane amounts of money.
The parking charges do annoy me to be honest, especially since they have risen $6 in the last 5 years, and in some cases, you're not actually parking anywhere near where you're heading. But as you say, the other option is just including it in the park ticket prices - but many would argue that then means those who arrive without a car are paying. The fact remains that Disney is a for-profit business, and whilst I'd argue they are generally one of the more ethical ones out there, they are still there to make money, so all costs must be covered. It's not a cheap place to go to. Parking charges are also just more of a "thing" in the US than we're used to in the UK, and other local attractions including SeaWorld, Universal Studios, Kennedy Space Center and more also charge similar rates for parking.
The other aspect of this is that the greater Orlando area gets VERY spread out once you leave downtown, and there are vast amounts of low-density "R1" single-family home development. These are the places people tend to stay when they are not staying at a Disney hotel, and sadly these types of development are a nightmare for any sort of transport planning to start with. Add to that the fact that building things in Florida is already a challenge to start with, and that for non-locals the heat and humidity can be oppressive, and as such you'll struggle to get people out of their (hire) cars and walking to a bus/tram/train stop, especially at the end of a busy day.
Even for the locals, SunRail has now been in operation for a bit over 5 years, having just been extended south to Poinciana. It has been heavily invested in with new or as-new equipment, brand new stations and lots of publicity, it goes through downtown Orlando, connecting with the central hub for the Lynx bus network, and many other bus routes and commercial and residential areas along the way. It still struggles to get more than 6000 passengers a day, and so far has only been able to support a weekday service - trials of weekends and public holidays fell completely flat. The Lynx bus network faces similar issues, and both Lynx and SunRail require incredible amounts of public money just to keep operating.
Improvements are being made in the area, both Disney and wider, but they take time and have to be carefully thought out - as the makeup of the area is such that many aspects of traditional demand modelling and urban planning go right out the window.