Because few companies / cities / etc do it without being regulated to do so. I would imagine in the case of Uber they will grant additional time for anyone who files with them the paperwork indicating they require an accommodation. Generally speaking if Uber extended the wait period to 5 minutes for everyone it would also be in violation of the ADA as again, they are not providing an accommodation for people with disabilities.
ADA exemptions exist for small companies where it would be unreasonable to bear the additional costs.
Fundamentally it's a political question of what kind of society we want, do we want one where cities have to make curb cuts in sidewalks or do we want one in which it is extremely difficult for wheelchair users to use sidewalks. The balance the US has currently settled on is that small business is largely exempt, and larger businesses must accommodate.
It should be noted that this case has not been ruled against Uber yet, and that the suit revolves around wait times, not wheelchair accessible cars.
To be honest once accessibility standards are adopted by society at large they are not very expensive in general. It is however, VERY expensive when accessible goods and services are not provided at scale.