There is nothing stopping you running a JoinMarket maker node, providing you have the technical capability to do this. It also means you leaving your Bitcoin funds in hot wallets connected to the internet at all times. In my experience, despite offering strong liquidity with 0 fees as a Maker, JoinMarket takers can not come to initiate a CoinJoin until quite some time later, it may be 24 hours, it may be 4+ days.

Wasabi is not instant, they join once an hour providing something doesn't go wrong which it often does, you need a minimum of around 0.1 BTC to participate ($5,600), whilst also leaving you with plenty of "toxic waste", which you are urged to either throw away or donate to others to then co-spend on your behalf [1] which given the high and increasing price of Bitcoin is very wasteful indeed. They also put incredibly low fees on the CoinJoins in my experience, with funds not confirming for 1 or 2 weeks.

With CoinJoins you do not jump the transaction graph. large CoinJoins are impressive things, but your funds can be traced, it's difficult, but overtime your anonymity set degrades, and certainly in Wasabi's case questions remain over how much they join their own coins leaving room for sybil attacks from authorities.

The value of a Bitcoin Mixer which did this, would be offering a user the ability to easily "dip in" to this forever spinning pool, which could have been spinning for the last 6 months continually. The user would get the best of both worlds so to speak, they would jump the transaction graph, whilst receiving coins which would always be anonymized from endless CoinJoins. Coins which were sent to the Mixer would also be added to this forever spinning CoinJoin pool. I think paying 1.5% to a service to provide this, on demand at 1 minutes notice, including getting coins Confirmed quickly is probably a fair price to pay.

[1] https://anonymixer.com/help/wasabi-change-coins