The main problem is the expected lifetime (and payout periods) of the two parts. Cooling solar panels brings more efficiency so it feels like a reasonable fit, cool the panels and get hot water. Its less efficient water heating because the panels aren't the ideal coating but they do still get quite hot and a few of them can definitely do the job. The big mismatch is in life.

A solar thermal heater typically lasts 10 years, its a pretty harsh environment and they have pumps and expansion vessels and water in the heat range of -25 through +80 C is just damaging over time. Where as a solar panel has a rated 80% output at 25 years. That mismatch is problematic for Solar panel payoff, 10 years is only just past break even point. Whereas the solar thermal heaters typically pay off within a few years so the reduced lifetime isn't such a big problem (but both are driven by local gas and electrical prices).

Companies are trying to do this better and products exist to do it. The added installation complexity and reduced expected lifetime are all a bit of an issue. Currently I suspect most people are better off with a dedicated solar heater and solar panels separately, at least for now until the water channels in the panels at least are reliable enough that they meet the panel lifetime and then the pump is installed in the roof cavity so its cheap and easy to replace. Work still needs to be done to work out the details and make the parts that fail easily swappable.