That is not an accurate statement.
Many lakes have concrete ramps, but the concrete has to end at some point. This is especially the case on northern lakes where ice and freezing/thawing make longer/deeper launches impractical.
Lake bottoms after the launches end can be rock, gravel, sand, mud, etc. Sandy and muddy lake bottoms are the most susceptible to propeller erosion. You can't design and engineer ramps to be propeller-proof. At some point, boat owners need to accept equal responsibility for using bunk trailers and for keeping the boat launches in good shape. Otherwise county and state budgets have to foot the bill, raising taxes, blah blah blah.
White Bear Lake in Minnesota is a great example. There is a great double-launch concrete ramp on the north end. It's a sandy-bottom lake. There is a propeller trench followed by a sandy hump about 10-15ft out that can catch lower units when maneuvering into and out of the launch. The county has had to fix the ramp repeatedly due to the power-loading.
Power loading is a problem in some places whether you want to accept it or not. Boating is not a right, it's a privilege. If you launch somewhere that is immune to the havoc of power-loading, that is great. Most boaters are ignorant of the boat launch conditions and how their actions affect the launches.