Several reasons.

First, a long-running SEAD / DEAD campaign by Ukraine last year and early this which thinned Russian air defence throughout occupied territories and left Russia with difficult deployment decisions for what remained.

Second, improvements to drones (speed and EW resistance) and expanded production - meaning the air defence can be saturated.

Third, Russia's vast geography itself, leaving countless approaches to targets impossible to defend.

"What air defense doing?" is apparently still an unsolved riddle.

Masses of drones. Some get shot down, but not all.

Bold of you to assume they have many defenses left.

Their lines of 50+ years old junk which have been rotting for decades because of corruption and are operated by low iq conscripts with no training?

A mystery really...

Saturation

Russian air defence equipment bad, organised even worse, and has been severely degraded over 4 years now.

The are largely concentrated around Moscow to defend the "brain" of the country, as well as personally Putin himself.