Obvious reasons.

someone has to do the discovery work. Either the fund does it (finds devs) or the developers do it (applies to the fund). There has to be a selection process somewhere. So either the devs can self-select or the fund can go searching through all Nostr projects on earth.

So it's about incentives. The fund isn't going to spend extra energy to ALSO give away money. Theyre already "spending" a ton. So the developers are incentivized to surface themselves up; to "spend" comparatively little (time) to make themselves known and potentially gain a lot.

The filtering and selection has to happen somewhere. "hey developer who wants free money, make yourself known to me by writing a little bit on this public form and I'll read all the submissions" is far lower friction and cheaper than "okay let me scour the whole fucking internet to find a place to lose a bunch of cash."