Its not simply creating new money. You should understand what a fork is in the first place. In most simple way, it is simply making two way out of one, for whatever reason.

Now in case of bitcoins, the fork brought about a new and similar currency called bitcoin cash. This didnot gabe you equal amount of bitcoing, but provided you with a newer crypto called bitcoin cash.

Now what happenes in a fork is that suppose a new thing is created, then this newer version has 0% value as compared to the older one which in this case is bitcoin.
So basically, the bitcoin cash got created but without any value, so in actual sense no new money was created from thin air.

Now this new thing/currency, with the changing market starts gaining value and hence now is when the actual money is being built or manufactured in a sense. Forks will come as long as people believe in the particular market, in this case bitcoin. Forks expand the market and creates newer monetary values out of nothing but the trust and popularity the former entity (bitcoin) holds.