It's frustrating to see engineering teams in 2024 spending countless hours optimizing their applications to run on what essentially amounts to 2008-era hardware configurations.
Start from the premise that the majority of developers out there have no experience dealing with concurrency, then consider that changing something's approach to concurrency/parallelism can occupy several developers' time for months or possibly years. Then consider that for a business the absolute costs of using 2-20x more instances than "necessary" may not justify the investment to actually switch to that. Then it makes sense why people so often choose to use concurrency = 1 (in which case you basically want the smallest instance size capable of running your app performantly) or whatever concurrency setting they've been using for ages, even if there are theoretical cost savings.
It's not hard, and it provides a nice managed service without the full complexity of running K8s.
And haven't used one again since moving off ...