Answer: Because your tires get hot. Factory street tires are generally designed for a wide range of operating conditions on the street, 36 to 42 psi keeps the tire cool and lasting a long time. But when you start pushing hard in the canyons or on a track, you generate higher loads, more heat and that air expands. If you start high, you end up with a rock-hard tire that has zero contact patch. You’ll slide right off the road. You have to drop that pressure so the tire can actually grow into the grip. If you don’t know the difference, you shouldn’t be pushing that hard.
There are street legal “track day” type tires or one can opt for some sticky track only rubber. For the rider that rides street plus occasional track days I recommend finding a good set of used wheels, keep a set of track tires mounted up and simply swap wheels when headed to the track.