Maw was not built every single game for the past 5 seasons

i use to build it all the time on top lane bruiser like before this season

Well yeah we used to have to choose between that and brutalizer. Now we dont.

well it keeps some burst mages in check and oppresses the weaker ones.

ex. Mages like anivia and brand (atleast as of patch 6.18) are decently strong despite the existance of maw, but nerfing maw will be an indirect buff to them.

So nerfing maw requires a whole set of other balance changes.

Of course people build it, otherwise mages would just be running around oneshotting everyone and their mother