Two big reasons!
1) You actually create more weeds. Most of the lake weeds you struggle with reproduce by either “fragmentation” or “seed banks.” When aquatic weeds are broken or cut, tiny fragments break off, sink to the bottom, take root and create new unwanted plants. Also, if aquatic weeds are disturbed by raking or cutting they will drop seeds, creating several new plants.
2) It is a lot of back breaking, messy work. The aquatic weeds you take out of the lake have to be put somewhere, (or leave them on the lawn and kill the grass). We’ve spent hours wading in the muck getting weeds out and hauling them away only to watch them grow back in a very short time.