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.
When everyone understands the advantages of decentralized cryptocurrency exchanges, and the exchanges themselves will have a more friendly and understandable interface, then the question of volume and liquidity will disappear by itself.
Hopefully we get there soon. 90% of the value of a DEX is that they are non-custodial. The other 10% varies by DEX, but users should figure it out soon!