They're a complicated problem, but I can't help but wonder if SpaceX underestimated the difficulty. They used to have a 3rd patty supplier for the helium tanks but they decided to manufacture them in-house a few years ago. As far as I know, all 3 tank problems have happened since they started producing their own.
The strut which failed in 2015 was outsourced. A tiny fraction of the struts from the factory failed at well under the specificed maximum load, and one of them ended up in that rocket.
Alternatively they're not incredibly hard / complex so not enough attention is being given to testing, manufacturing, etc. Though I'm curious which way it is.