I thought why is qemu used here? Why not use linux native namespaces and cgroups.

Permissions. Isolation.

so, like namespaces and cgroups?

	▲	figmert 43 minutes ago \| parent [-]
		Android kernel has the relevant kernel parameters disabled. It is entirely possible to run containers directly on android, but it requires enabled the relevant parameter (iirc no recompilation need, just a cmdline change). But this of course requires root.