A DMA cheat requires a hardware change (and a second device). That is a much higher barrier than a download plus reboot.
> you can achieve the same with user mode anticheats
A user mode anti cheat is immediately defeated by a kernel mode cheat, and cheaters have already moved past this in practice.
A user mode anti cheat (on windows) with admin privileges has pretty much full system access anyway, so presumably if you have a problem with kernel AC you also have a problem with user mode.
Lastly, cheating is an arms race. While in theory, the cheaters will always win, the only thing that actually matters is what the cheaters are doing in practice. Kernel mode is default even for free cheats you download, so the defaults have to cover that.