Off the top of my head, the major reasons are:
1. Artistic expression - Probably the most important one. A chance to breathe life into the mechanics by sharing your particular vision of what the game world looks like. Maybe they want an anime aesthetic, anthropomorphic woodland creatures, friendly and inviting CalArt style, a dark and horrifying inky look, etc.
2. Stable platform - If you plan on producing lots of adventures and supplements, knowing the game is completely under your control is comforting. Nothing worse than the original creator revamping their game to make all your material obsolete. Also, it only takes one alarming social media post to put creators in hot water... and those that support them.
3. Dialing in perfection - On closer examination, a lot of those "similar" systems actually do vary quite a bit. It might not be noticeable as you're flipping through your first time, but taking out certain spells (such as resurrection magic), allowing unique combat maneuvers, capping level progression, etc., can be quite noticeable in play.
4. Compatibility - Thanks to systems being similar, potential customers might be inclined to pick up your game just for your unique tables and houserules. If you're producing more product, they might skip your rulebook, but pick up your adventures to play in their own game. Not uncommon to hear someone using one game's skill system, another's initiative rules, another's weapon tables, etc.
5. Community - If your game resonates with enough people, a strong community can form that exchange ideas and grow the game around your particular vision.