"So you have an unique skill, that no one know what it does, and make you do action while unconcious at a better rate than normal people? Yes, we could invest magic stones and such to make you grow and see what that does, not that we are in any difficult financial situation or weak enough to be able to make you grow just a couple of more level, but no, better to just hate on you."
Literally, many times the story has shown that magic stones are literally exp bottles. To achieve enough points for its third level MC had to reach only lvl11 (and presumably diverting part of those "status points" shown in the first chapter in other skills, to be able to hunt alone). The only opposite reason I can think is what the guildmaster says in chapter 6, that "if you intake too much magic essence from the magic stones, you will burst out". But that doesn't seems to be an absolute limit, more like in a restricted time. Even if it was the former, they could have probably just paid someone to carry him for some more levels. Are unique skills so common to not be a potential valuable investment?