It's cheaper than most options where you need to pay a human to manage something with the same level of scalability and security that AWS provides.
1000 lambdas with 1000 database connections is also going to be a horrible way to do things on the backend. Add in the cost of RDS proxy for lambda and the ECS solution becomes even better.
I personally don’t think running on EC2 is particularly difficult to manage, and it’s even cheaper again.