Custodial wallets, exchanges even gambling sites are the main reasons why the fees are highly volatile and insane when the required fee is very low so its like some kind of manipulation too. Every exchanges and sites should move to dynamic fee structure so they can avoid spiking the mempool fee with their high fee which is win-win for them as well as to the crypto community.
Fee rates going up and down has nothing to do with "manipuation" by custodial wallets, exchanges or gambling sites.
It is simply a function of the "supply and demand" nature of the Bitcoin network. Space in bitcoin blocks is limited, so if you want to get your transaction in on quickly, you need to "outbid" others trying to get in.
For instance... if there are 1000 transactions in the mempool (that will completely fill a block)... and there are 999 transactions that pay a fee rate of 1000 sats/vbyte and 1 transaction that pays 1 sat/byte... and you want to send a transaction and get it in the next block.
It makes no difference that 999 transactions are sending with 1000 sats/byte... you would only need to outbid the 1000th ranked transaction to likely get in... in which case, you could use 2 sats/byte and you would "outrank" the 1 sat/byte transaction.
What causes problems is that often, we have a situation where there are just a lot more transactions being generated and sent than can be included... and a lot of the people sending these transactions, want them confirmed fast... so they start outbidding each other to try and get it the next block.
THAT is what causes fees to go up... not anything untoward by exchanges, gambling sites or custodial wallets... if anything, they usually end up sending with
low fees to try and minimise their expenses (and increasing their profits).