My tinfoil hat theory is that making local development difficult is the goal rather than a side effect. Why not just run the stack yourself if it's that easy? Better for Amazon if the development process for using their services involves as many other AWS services as possible.
I wouldn't put it past them to do something like that if that were indeed going to work out, but the more reasonable thing is that they're just bad at it. Lambda was weak for years. Also, a 'local stack' for testing probably wouldn't cover nearly enough basis for people to 'just to it themselves'.
Literally today I had a billing problem with AWS and it was one of the worst, broken UI experiences I've ever seen. Involving money to boot (usually those things have higher standards).
They are really bad at quite a lot of things, it is what it is - there might be some rhyme or reason to it, for example, they might not care about documentation, or may be much more focused on bread and butter ec2, bit it's totally plausible that there are a lot of frayed edges.
You know that difficult-to-attribute quote: 'Don't attribute to malice that which can just as easily be attributed to incompetence'.
