That simply reflects, in part, the cost differential. It's difficult to find the most recent data, but as of mid 2025 and, AFAICT, still today, Ukraine and Russia were still exchanging 10,000+ shells every day. Moreover, Russia fires 5 shells for every 1 Ukrainian shell. It used to be 10:1. It's a huge reason Ukraine can't break through Russian lines, and during offenses hold off Russian advances. Ukraine still has major supply issues with shells, and that's also likely one reason for their emphasis on drones. It's certainly the reason Germany and others are still pursuing increased artillery production.
Ceteris paribus, the marginal effectiveness of deploying more drones may be superior to more artillery, but that's against the backdrop of existing artillery usage. If Ukraine switched to only drones, they'd lose the war in weeks if not days.