Not saying it's a good excuse, just a partial explanation. If their dealer and distributor network in the US was stronger, they'd know how to get the parts from Europe. It's the middle part of the equation that is weak. We were on Lake Norman in NC with hundreds, perhaps thousands, of Volvo I/O's, but solid dealers. I would say parts availability and cost was at worst 10% worse than Mercruiser. As the need for parts was probably 15-20% less than Mercruiser, it pretty well evened out.
Now some hints that people may or may not have tried and may be better than your local dealer, depending on your location. These are from someone at my home today with decades of Volvo experience. Volvo's online parts store.
Volvo Penta Marine Engine Parts and Accessories Online Store - volvopentastore.com In FLL, Diesel Services of America is one that unlike most others doesn't treat Volvo as a secondary part of their business. Find one in your area that doesn't. If their web site lists every other brand and doesn't highlight Volvo, then less likely to help. Also, find one willing to deal internationally and do what it takes to find the part. Again, we have a big advantage in South Florida that obtaining parts from Europe is commonplace for many. Now the shipping options from Europe may not be great and you'll have to make a choice on speed vs.cost.
In South Florida there are a lot of IPS boats, so Volvo is becoming far more common. Unfortunately, that doesn't necessarily help those with legacy engines but it does indicate more hope in the future.
On the lake there were many legacy gas Volvo's and the dealers there somehow got the parts.
I'm not by any means saying the problem isn't there though. It is. And it's far more of a problem in some areas than others. One reason we said no to Volvo IPS was we were going to be doing the loop and knew that the number of yards with Volvo experience would be far less than with MAN and conventional shafts. Of course with Cummins or CAT there would have been even more knowledge, but we went middle of the road with MAN.