An FPV drone costs a few hundred dollars; an interceptor that shoots it down quickly costs hundreds of thousands; the aircraft it can disable costs tens of millions. This arithmetic structurally favours the attacker.
Example
FPV production grew from roughly 20,000 to 200,000 units per month within about a year — a tenfold increase. Shahed launches grew from ~130 to over 1,000 per week in a similar timeframe.
This scale and cost asymmetry are why conventional, expensive air defence cannot sustainably keep pace with the domain — see Chapter 6.2 for what does and does not work against it.