This happens only because Intel went much further than AMD and other companies in exploring these effects. If other companies used speculative execution as much as Intel, the result would be the same. It is not a flaw of implementation, it is a flaw of basic design.

They have different cache architectures; Intel uses inclusive (i.e. all levels contain keys from previous levels), AMD uses exclusive cache (each level contains unrelated entries to any other level). This might have different effects on classes of vulnerabilities they are prone to.

I know that some AMD CPUs a long time ago had exclusive caches, but for Ryzen, I'm pretty sure that both L1 and L2 are inclusive, and L3 a victim cache.

the inclusivity types don't really play a role in these types of attack (until you get to a very practical stage where this might matter), not least because there are other sidechannels that can be exploited. an inclusive llc is just convenient.

that said, some newer Intel LLCs are non-inclusive, and amd changed its cache relations as well in ryzen

The focus is on Intel partially because it's the most valuable target, so we might be looking at selection bias.

That's really not the case. Once you know of an attack of this sort it's pretty easy to test everyone's chips for it. Bascially every Intel chip since Nehalem is vulnerable to Meltdown, as are IBM's POWER chips, as is one out-of-order ARM core but not any of the others. And we know that AMD chips are safe from that vulnerability.

Whether you run security checks in sequence or in parallel with a data access is pretty fundamental to the design of a core. Doing the later is has performance advantages but it's really hard to verify that won't be any architectural leaks as a result, let alone these micro-architectural leaks that nobody was thinking about.

I agree with everything you say, but security researchers will still prefer searching for new vulnerabilities in Intel (which is hard, I hope we agree on that) and verify they don't work on AMD, rather than the other way around.