He doesn't really have time to stick around to enjoy parties. The clock is ticking. Every day counts. There are places to see out there.
If it was that simple, he would be doing it. The author hasn't really explained why. Perhaps draining it in sufficient quantity (through any means he knows) causes damage of equal impact. Or draining it doesn't even matter anymore if Yona absorbing/generating the mana already causes the damage. Kind of like if you have a battery you keep charging with far higher power than you should, like by using the wrong charger. So, even if you kept draining the battery to avoid it reaching 100%, the charging power level already causes damage.