Accessibility Tools on StackExchange sites

I am taking an HCI class this semester at school and one thing that is glaringly obvious is how the design of current online platforms is wanting in certain areas such as accessibility.

Why do a majority of the websites including, the StackExchange ones, do not have basic accessibility tools such as color inversion, speech input/output etc?

I know there are tools (Browser extensions, screen readers ) that facilitate this, but putting some of those on the website itself would only improve accessibility and ease of use for everyone.

I was wondering if there are any implementation issues with respect to design, uniformity, feasibility, effort-result tradeoff from the perspective of the platform, which is why these tools that (obviously) should be there, aren't there?