Why does no one fix eqnarray?

Fixing it would have mostly disadvantages.

Firstly for any serious math use you probably want amsmath anyway, that adds far more than a better eqnarray and if you are loading amsmath and not using eqnarray then fixing it has no benefit.

eqnarray has been the way it is for 30 years and the main thing wrong with it is over-wide spacing around the = in that time many people have fixed that in various ways many of them equivalent to ...\!&=&\!... If the spacing were changed these documents would get no warning but the left and right sides of the equation would start over-printing the relation. It is not clear that everyone would see that as an improvement.

LaTeX can not absolutely guarantee that a 30 year old document processes without error but we would not make a change that knowingly broke such documents.

I vaguely remember reading that Donald E. Knuth has frozen the codebase except for actual errors in function that cannot be worked around reliably. His versioning system is a key to what his thoughts on this matter were, his popular projects have version numbers that approach (but can never reach) various trancendental numbers.

Then as mentioned there is no point in rocking a seaworthy boat.

Also we should try and get in Donald's head where he offered bounties for finding errors in his books (of which he paid out some $2.56 cheques of which most were framed and not cashed). He was a pragmatic perfectionist. Do what need to be done to get the job done but no more, the version numbers would keep getting longer if the code was tinkered with endlessly which will discourage random tinkering.

TeX was just Donald's way of getting presentable typesetting for his own books that could no longer be served by Hot metal typesetting and hand tuned equations. We should be thankful that he has had a lot of foresight many of us would have run past blindly.