I see many DEX (Decentralized Exchanges) like Airswap, 1inch.exchange exchanging altcoins, but not offering BTC. Instead you should exchange first to WrappedBTC or any kind of BTC token.
Why isn't possible to exchange directly to BTC or are they any DEX supporting swap to BTC directly?

The problem is the bitcoin network needs more time to be confirmed and it can't be done instantly. The majority of swap services only accepted coins other than bitcoin caused by these coins have enough scalability to do an almost instant transaction. You can take a small explanation from blockchain[dot]com as your reference https://support.blockchain.com/hc/en-us/articles/360023819791-How-long-will-my-Swap-order-take-