Say you’re trying to defend against something like a Shahed-136. It can hit pretty much anywhere in Ukraine. You can’t stick an AA gun on everything that Russia might consider trading a Shahed-136 for.

As far as I know, the routine in the current war is - the AA gun is on a truck that moves 80 km/h, the drone comes in 300 km/h, one or many truck crews position themselves on likely vantage points for intercepting, and the rest is luck.