I read somewhere (
here - found it) that Shimano made a deliberate decision not to try to rapidly ramp up production because they see the current boom as transient and not sustainable. If it turns out they are wrong, then their market share will be eroded a bit. However, if they are right, they will still be around in a few years time. I think they do have new production facilities in progress, but that was already on the cards. I expect they are working to a long-term plan rather than knee-jerk response to a (possibly) temporary uptick in demand. Time will tell. I'm sure Sram are not crying about it at the moment.
That is definately part of the issue, up turn in demand, but the other side is that most of Shimano's factories are in China, and therefore raw material, slow working due to power restrictions, transport restrictions, and a even political issues are at play. I build clean environments for chip manufacturing, there is a world shortage of chips, so many industries have to slow down production. My clients are at full production but they are not jumping over themselves to build new fabs. Everyone sees it as a short term issue maybe 2 years.. but expansion needs long term business. That is not assured. Yes, there are new fabs being built and others planned but that was planned. Also the move to pull production out of China has been going on a long while... but will take a long time before that makes a dent in the supply issues.
What does need asking is why this was not foreseen, all the high paid economists and governments were too slow to see the warnings and react.. so with all our technology and AI... etc etc etc... snafu seems to be at play as normal. And of course its not just bikes and bike parts, but every manufactured item imaginable.