Because we can easily tutor up any creature we want, creature chains like Pestermite into Breaching Hippocamp will allowing us to move from a two-drop creature to a five-drop creature all in one turn
Paying 5 mana protects all your creatures, with even more lifegain
If you can generate early creature pressure and force opponents to fight on those terms, protection spells (Gods Willing, Akromas Will) will grant you a powerful instant interaction game that can lead to lethal blowouts
Besides, Mew has taken a noticeable dip since Arceus/Palkia, but I don't know if stupid powercreep necessarily indicates it's a well designed deck #16 *laughs in drapion & mightyena #17 Yeah the deck's annoying but I really hate card games gatekeeping players from playing due to price
Its really popular, even just to help bring your creatures back, but also if you care about sacrificing or similar, too