It’s probably because they make a great product.
However, not everyone wants to spend $200 on a suspension training system.
For those DIYers, I present the following videos.
The lashing straps that I bought came in 13′ lengths, so I cut them down to 8′ as per the video and used the extra lashing strap (in lieu of the rope) to make foot loops. My entire cost including Quick Link (not a carabiner) and the PVC tubing was $35.
That’s less than 1/5 the cost of an official TRX suspension trainer.