Its not about reversibility.

Let's say you and me trade BTC/XMR.

When we start the trade, both security deposits + the trade amount in XMR get locked.

If I'm offering the BTC, then if I never send the BTC you just never release the coins. In this case you call the arbitrators, you prove you never received the coins, they will unlock the funds and since I am trying to scam you, you will get a refund of your XMR, your security deposit, but also my security deposit, so I lose the deposit.

If you are offering the BTC, let's say you send them to me but I claim I never received them. Then, again you call the arbitrators, you prove that you sent them to me, they will unlock the funds, and you will receive either your security deposit, or that PLUS my security deposit, or maybe even the whole trade amount (which were my coins, but I tried to scam you, so I lose them)

So the security deposits are in place to try to make it unprofitable to scam people in this situations.

Actually, since a bank transfer is reversible, security deposits do not really give safety in this case. We can do a trade normally, I send you 300$ and you send me 2 xmr, and I can contact the bank 2 weeks later saying someone hacked me and get my money back while I still hold the xmr. So this is always a risk. That's why cash is always better, but personally I have used haveno a few times and never got problems.