Oh wow. We have a test tenancy that's carried throughout production, so you make requests against real backends (read/write data in the test namespace, sometimes read-only production data). There's a proxy in front doing rate limiting, endpoint whitelisting, audit logging, emergency lockdown, etc. I never thought of deploying a whole separate environment just for integration testing.
Still, seems you could keep a handful of integration test environments always running? Time spent waiting your turn for one of them could well be less than time spent spinning up a whole bunch of servers.
There is an effort to make everything hermetic. Namespacing is hard, but not always possible, and touching production servers (and potentially crashing them) could cause significant revenue damage.
I don't think all tests should be hermetic - the effort to make such things happen usually do not overweigh the time it takes to do them, but hey - that's what we are doing.
Still, seems you could keep a handful of integration test environments always running? Time spent waiting your turn for one of them could well be less than time spent spinning up a whole bunch of servers.