Not all disabled people are eligible for fast passes, too.
Obviously every company is different and some are better than others, but a lot of disability access schemes are created by non-disabled people who don't know anything about disability and expect disabled people to jump through hoops to "prove" that they are disabled.
A lot of places including a some of the big amusement parks require the 'Access Card' but you have to pay for that, and the Access Card scheme doesn't accept medical letters as proof of disability (their website also says they don't consider blue badges or ESA as proof of disability). Many disabled people don't meet the Access Card's extremely strict criterion for evidence.
I know a few schemes where you need to be a blue badge holder in order to be eligible, but to have a blue badge you need to be medically able to drive a car and be able to afford a car (or live in a household with a car and a driver). If you have a disability than means you're medically not allowed to drive, then you're fucked.
Other places assume all disabled people are on benefits and don't work (which is bigoted), so if you're a disabled person who does work and who has chosen not to apply for benefits, you're not eligible.
The theatre company Punchdrunk asked me if I was "registered disabled" (which does not exist; there's no such thing as a "disability register" in this country, and the concept of disability registers have a very frightening history) and another disability activist I follow on Twitter was asked for her "registration number from the disability register" before she was allowed to book a wheelchair space for an event.
Parents with autistic children who are still fighting for a formal diagnosis (even getting an assessment can take forever) might also not have "proof."