Why would that be, that drone warfare advantages defense?

For an offensive action you need to concentrate enough forces and material in one area to achieve a breakthrough.
In Ukraine, there is now a "grey area" between the fronts which neither side controls but is permanently patrolled by drones, similar to WW1's "no-mans-land".
It's up to 20-30km wide, and any vehicle entering it is destroyed by drone swarms.
Crossing it on foot takes up to a week, and less than 10% make it through alive.
This makes it impossible to bring heavy equipment to the actual front line for an assault, or even re-supply your soldiers hiding in trenches there.