It's partly due to mowing alongside highways and medians. That is primarily motivated by keeping the number of small animals down, reducing fire risk, and other reasons, but fewer tall grasses means drastically fewer bugs. Also, pesticides and general global insect decline. This is not primarily due to aerodynamics.
It's smart to keep it maintained for safety reasons, but that doesn't explain the full story - there's bound to be plenty of unkempt grassland and whatnot further away from the road where insect populations SHOULD be thriving.
It's partly due to mowing alongside highways and medians. ... fewer tall grasses means drastically fewer bugs
This is way too general: in some countries and alongside specific highways, the state of grasslands is actually really good. Like: good enough to grow rare orchids and having >10 or >20 plant species/square meter. Because of correct mowing regimes for that vegetation type.
It also lacks nuance because as explained in another comment already: not mowing at all can make things worse. Which brings us to the tall grasses: meadows featuring a lot of tall grasses but hardly any other flowering plants have can have much less insects that meadows with a coverage of only like 20% grasses where the rest is taken by a diversity of flowering plants.
