Because a root can already be an obstacle for an excavator, depending on its size. In any case, it hinders the workflow, as it either has to be dealt with beforehand or the excavation work has to be interrupted in between. It's not like he digs out 20 buckets of earth, then takes the root once and throws it to a corner of the property, and then continues loading the truck with soil. Loading the truck is not free either. Of course, one can discuss whether a few euros less would also be possible, but clearing a property of roots also takes time, even with an excavator. On the other hand, the contractor also has to see that he has a few positions where he can make money. Pure excavation brings him almost nothing, just like the installation of recycling/gravel, etc. Only with the sewer work is there still some money to be earned, but only if it runs smoothly, meaning the connections are where they should be, no cable gets damaged, and the workers are reasonably good. In the past, this balanced out a bit because most companies (at least the smaller ones) had 30% private individuals or small orders, where 5-10% more was possible. Today, however, this is no longer possible given the order situation. Most craftsmen have no financial advantage in the current construction situation, as prices do not get better for them, or they are so squeezed by the GCs to fulfill all orders that in the end even less remains in their pockets. The only advantage is that the order situation is secured. Most of the money stays with the GC.