The reason for trading limits is to reduce the damage in case of chargeback. Chargeback is also why paypal is not accepted at Bisq.
Most P2P exchanges use reputation systems that might work most of the time, but sybil attacks make it easy to gain fake reputation, and that’s why they usually allow as an extra measure a kind of KYC. Bisq uses security deposits (main reason is that collateral is more private than reputation), and for those methods where chargebacks are not easy but possible, account signing, which is a integrity verification (the account data you filled at Bisq is the same you’re using to pay) because the main reason that these payment methods justify for a chargeback is a stolen account.
If someone steals an account, he won’t like having these limits and probably will go somewhere else, he needs to empty the stolen account quickly.
These are disadvantages of working with unreliable payment methods. Would Bisq onboarding and trading volume improve if there were no trading limits? Yes, but it would be at cost of security. Before account signing, Bisq had a couple of chargebacks and it’s something very annoying. Nobody likes trading limits, but chargebacks are worst.