It's the same situation in London, only about 25% of Underground stations are accessible[1]. This seems to be the cost of old infrastructure - when most of the network was built, accessibility simply wasn't a consideration. Neither was having a decent mobile signal, or air conditioning. Fortunately, we've learned from this and newer systems like the ones in Shanghai and Taiwan seem to be far more accessible.

[1] https://tfl.gov.uk/transport-accessibility/wheelchair-access...