In a lot of cases we don't even have a good cause of death. There are a lot of areas without a good medical examiner system and the systems are overloaded even when they normally are good.
Covid is quite capable of killing with a clot when your symptoms aren't to the point that you'll be in the hospital these days. This will show up as a stroke or a heart attack or pulmonary embolism--and if the docs don't put in the effort they won't know it was Covid (and for that matter it can't be proven anyway--lethal clots happen even without Covid. While it's a reasonable presumption that Covid caused the clot there's no proof in any given case.) There's also a lot of families that don't want Covid on the death certificate--even more reason the clot won't be attributed to Covid.
(And, yes, there are plenty of other ways clots can kill--I'm just looking at the cases that are likely to kill without ever reaching the hospital.)
Undertakers have also been noticing this--lots of bodies showing Covid clotting even though they aren't reported as Covid deaths.
Edit: I forgot an additional factor. If you do survive your Covid hospital stay you have a substantially elevated all-cause mortality rate at least for the next year. AFIAK the mechanism has not yet been identified.