@Freestyle: Jealousy + prejudice.
Since he never used his points until now (saving them to level his Restoration from lv 9 to 10 that gave him the OP version of it) to the eyes of others he's been:
1. A cripple (left hand didn't work)
2. Could only restore items (the lv 10 made it change to 'targets')
3. Can only fight the weak slime.
So to them he's just a weak guy who hunt slime for money.
Then suddenly this one new cute girl show up with him and suddenly the two are farming a shit load of monsters.
So to them, it's clearly the girl's ability that's making them able to hunt this well, after all they've been seeing him for 25 years so he likely don't have any ability to hunt that well.
@Riger: He can, and was going to until Sora told him she prefer him staying like this (he's cooler this way)
And what is a boy to do when his tiny onee-san demand he remains a dandy old man?