Even when people started speculating (ha!) that speculative execution could be problematic it took years before they managed to exploit it.
Are they really? I don't have an extremely deep understanding of Intel's implementation of x86 ISA, but I do know enough to say that so far we've been able to effectively mitigate almost all of these attacks with existing instructions available on the Intel CPUs. That doesn't mean that they are still not open to other variants of these attacks - but at some point you have to assume diminishing returns. Spectre is still very difficult to exploit, for example.
Perhaps this has little to do with Intel and more to do with software authors cutting corners? LFENCE and SFENCE are reasonably well documented, after all...
https://www.theregister.co.uk/2007/06/28/core_2_duo_errata/
The scariest part is that many of the best security minds work for various intelligence agencies. They very likely have known about such things for a very long time.
Meltdown strikes me as an almost perfect vulnerability. It affects almost everyone. It is undetectable until exploited and once exploited, it immediately goes away until the next time. It's easy to keep secret. Most importantly, it's a one-way vulnerability. Keep your secure systems from running untrusted code and there's zero risk. Since this is standard protocol anyway for those systems, you don't have the risk of someone running across a code patch somewhere.
The only potential downside is that the juiciest targets also aren't running untrusted code (though most foreign affairs workers probably run untrusted code). The big point of interest here is information symmetry. In most cases, giving others secret information is bad. In this case, both the best and worse case situations work out well for the USA. If nobody else knows, they get free info. If everyone else does know, then everyone gets perfect information about everything. This favors the most powerful country. They can eliminate the unknowns (the only real danger). In contrast, knowing you are going to be crushed does nothing if you can't hide your own hand either. So, the best case is very good and the worst case is still acceptable.