Because they’re not? There are 4K 8 Megapixel PoE Starlight sensor webcams on Amazon under $100, and here’s another variation on the above for $33. It claims full 4K with an appropriate monitor.
https://www.amazon.com/Computer-Privacy-Streaming-Microphone-Conference/dp/B08CSMW9RQ/
【4K Web camera】: Thanks for Sony CMOS image sensor, this 8mp webcam can supply Ultra HD Video Resolution 4k at 20fps (3840x2160P) when connect with 4k LCD screen/monitor. [Please note: The Computer camera also does 1920x1080p video at 30fps as 1080p webcam， when connect with 1080P LCD Screen/monitor]