Because maybe they'd rather at least get something for him than let him go for free to Bayern. But at the end it's Goretzka's decision where he wants to go. Unless they can convince him to renew his contract which is probably not very likely only thing Schalke can do is to keep him until his contract runs up. If they'll decide for that then they'll demand crazy money like 50M+ in January that no club will want to pay.
You misunderstood my point. If Schalke would sell for 15M then why wouldn't Bayern put in a bid as well? 15M is not much for one of the top German prospects. And if Schalke refuses to sell to them but not to us that would only work for them if Goretzka prefers Barça. And if Schalke are in danger of not making European spots I doubt they'd sell their best player for that amount. Losing out on 15M is probably more acceptable than losing out on European money and competition.