Probably cost and liability. You block a cell phone then there's the possibility you're blocking a cell phone trying to call 911. There have been days I've dreamed being able to go to a movie theater and enjoy it like I did as a kid. No people on their bright touch screens in the dark. No one forgetting to turn their ringers off or even worse answering them...
https://www.wikihow.com/Make-Your-Own-Cell-Phone-Jammer
Well they'd still check their device. They wouldn't likely get a signal though. I'm a bit confused how these can be purchased online but are illegal as all hell to use.
Honeypot maybe?
Hey, they used to sell cable descramblers in both kits and online before cable companies got more sophisticated. I always bought 'em and showed many others. Those were illegal, too - LOL
That it is ... I always wondered why they didn't just jam cell phones entirely in prisons.