That's not obvious at all. For cars to be accessible, the car needs to be designed to be accessible. For public transportation to be accessible, it needs to be designed to be accessible. The situations are identical.

Here in the US, all public transportation is required by law to be wheelchair accessible. Perhaps Europe is different, but that is a decision that they have made, it is not an inherent limitation of public transportation.

Cars are not inherently wheelchair friendly; they must be designed to accommodate them. Trains and buses are exactly the same. Here at least, public transport is much more accessible than private vehicles because all public transportation must be wheelchair accessible, but there is no such requirement for private vehicles.