Decentralized exchanges do not have enough volume, but this is due to popularity. When decentralized exchanges become more popular, then the problem with small volumes will be solved by itself.
They need to earn the reputation to be popular.
Binance has a plan to launch their decentralized exchange this year and I guess this is a big news that would certainly affect the decentralized exchanges
in a positive way. when people are satisfied with this upcoming decentralized exchange, it would easily result to an increase of volume.