When he was with a larger company, cisco and juniper were the only options.
I see tons of small businesses (mom & pop, restaurants) with a UI AP or two, of course, but that's not what you meant, I don't think.
The places that COULD use UI often just don't care, and want the cheap toilet paper (netgear, ebay whatever) since cost discipline can be critical. I think there's a niche of biz with enough margin that networking/cameras get sold together and they don't insist on lowest price. My guess would also be the MSP that is quoting the job also heavily influences whether UI is used, plenty of dinosaurs out there.
Bigger places want routing, network virtualization, etc from the big players you mentioned. UI doesn't want to mess with BGP, spine-leaf, sd access/wan, etc. There's also like the 24/7 support options they want, and access to the partner/VAR/contractor networks so you have tons of options. The sales deals and dinners unfortunately factor into this too...