There are a few reasons

They can only produce so many weapons a day, and use most of them quickly, while saving a small stockpile for mass strikes.

Strikes on Moscow may be “symbolic”, but from Ukraine’s point of view, it’s better to spend limited resources to hit a lesser defended but still important target first.

It takes time to actually find a target, and then find/take out/exhaust/bypass defenses (which constantly shift). Many drone strikes are basically intended to “feel around” for where defenses are, aren’t, or are light enough to saturate.

Rather than trying to save up expensive large warhead weapons to try and saturate prime targets in a heavily defended areas, Ukraine is slowly picking away at Russia’s air defense assets, weapons production, and logistics. Russia now no longer has enough defense for complete coverage, and is forced to prioritize certain areas for protection. This means Ukraine now has a much easier time hitting still very important targets outside of Moscow. Ukraine and Russia are now sending hundreds of drone strikes every day at each other of various sizes, with occasional bursts reaching into the thousands.

We are now reaching the point where Russia can’t even fully protect Moscow, which is why you see an increasing amount of high profile hits on it.