I think a lot of laptops do this because their cooling system is too weak for the CPU.

The Lenovo Thinkpad t480s comes to my mind: with factory settings, the CPU was unable to reach its advertised boost frequency nor maintain its max non-boost frequency for more than a few minutes. After 5 to 10 minutes, the machine was usually running at 300/400mhz under its advertised clock speed.

Undervolting helped a lot but could not fix the issue completely, under clocking was the only solution to maintain a consistent clock speed.