[SeanOC]

They look into it every few years but so far the costs to do so have been astronomical so it doesn't go past studies.

I don't have a good reference at the moment but from what I understand, most of it comes down to it being much harder to retrofit that kind of thing on to a 100+-year-old system than it is to build it into a new system. More specifically, since nothing was built with that in mind there are all kinds of challenges that would be very expensive to address like not all platforms are level or straight (imagine the cost and disruption involved in expanding and rebuilding the 4/5/6 platform and tunnels at Union Station to make them straight) and not all trains have doors that align in the same places (different generations of trains in service at the same time, different designs of trains on different lines, etc).

None of this means it can't or shouldn't be addressed eventually, but like many things in life there are reasons things aren't "just done" and it's important to understand those reasons if you want to make change happen.