It would appear this activity is from Unisat which allows for wallets, transfer and minting of Ordinals & BRC20, although I'm not 100% certain:
https://unisat.io/The motivation is purely to profit.
I think the reason for there being only one entity handling most of the Ordinals transactions is because Unisat is the only wallet to provide such functionality. If there were other wallets and/or exchanges doing this then I'm sure we'd see a mich higer ratio of public keys involved.
It's all really just hype though, and it's eventually going to fade into irrelevancy just like ICOs.