However when depositing or withdrawing ETH, USDT or USDC. I can only send via Ethereum (ERC20) network. Is there a reason to this because i can't figure out why i couldn't use BEP20, TRC20, Arbitrum One, Polygon (even though you can deposit MAtic with it), or others.
Ethereum is a biggest altcoin and its blockchain is the best among many altcoin blockchains. I don't compare ERC-20 chain with Bitcoin blockchain as Bitcoin is unique and the best but among abundant available altcoins, Ethereum simply is the best altcoin.
Other altcoin networks can have congestions, overloads and security problems too but Ethereum is better. Of course you have to sacrifice convenience, comfortableness and cheap transaction fee for OPSEC.