Because with certain Algos like Dagger Hashimoto or whatever that algo was called for ETH had a crappy build in miner called ethminer.

I used it in 2016, it basically made all my rigs freeze during the epoch change, there was also no monitoring, no dual mining, etc.

I rather pay claymore the 1% fee for the stability than deal with ethminer. Sure it was closed source but he had a reputation and they were all on rigs with no private keys or anything.