mag is a part of a verb called mögen which is a modal verb. There are 6 modal verbs and they are all irregular.
When conjugating "mögen", why is it "er/sie/es mag" rather than "er/sie/es magt"?
It's an irregular verb. The conjugations are different from what you'd normally expect.
Because it is an irregular verb
Ich mag Bier
Und ich bin 70% wasser