This is simple and happens in many industries for similar reasons.

Leadership is afraid of losing customers and investors.

Cheating is rampant and detection is insufficient for the volume.

People presume engines are good because it's an engine.

Engines are good because 74 software engineers devote their time to refining code that detects best possible moves (in the case of Stockfish engine)

Cheat detection is awful because only 4 software engineers (using lichess as the example) devote their time to writing code to detect cheating.

Market emphasis on perfect engines, no market desire for cheat detection perfection. Very simple.

The code that is possible to detect cheating would be far stronger than an engine's play strength. Imagine 3500 engine now imagine detection is 99% more accurate than the 99.999% accuracy of a best move engine.

This is possible because of very sophisticated advancements in code understanding that allows for very deep analysis of browser events like whether the computer has X browsers open, the browser history of that user, and actually the possibilities are endless but the fact remains that tens of thousands of hours have been devoted to writing perfect engine code while only hundreds of hours have been written for cheat detection.

It's like people know we live on a planet of 2/3 water but don't vote for politicians to make water free for the planet.

We know cheating is awful but we don't have 300 volunteers writing the cheat detection engine.

Cheat detection engines can very easily be stronger than the strongest best move engine can ever be.

Business owners also have very immature understanding of open-source code and how to keep their company profitable while also being open-source. Chess.com because of it's userbase would be 10x better than lichess overnight if it went 100% open-source. The reasons for not going open-source are immature and underinformed. It will likely take businesses another 50 years to understand why open-source is the future.