Take this into account too.
Go to a golf course on the weekend and people wear polos. Nice ones. Granted there are public municipal parks where guys are out there in an old beat up polo, but it is usually a golf shirt nonetheless.
I go to USTA tourneys and see guys in soccer jerseys and everything else. Show up to a class or drill in a fed shirt and you look silly. The $15 practice tee works best or even a nice v-neck. I imagine the golf apparel world is vastly bigger let alone the hard goods side of it.
The only thing that tennis players really buy a ton of is shoes.