It's like movie sequels. If the first batch of movies sell so well and make such a rabid/loyal/willing fanbase, then of course the company is going to make more sequels if it means more $$$, even if the sequels are terrible. However, not saying the recent Pokemon games are bad; not at all! They're good. But they keep making more due to the fanbase and of course, more profit, and in the process they ruin out of ideas for not only a "story" of sorts, but new Pokemon.
As for the Legendaries... I've never honestly cared for them

Most of the newer ones look anywhere from meh, to silly, to ridiculous. I usually stray away from them