Because this is not an "Armslist" where people sell gun.
It is a forum for the exchange of information, in which some members trade/buy/sell guns.
Not all new people are here trying to just trade/buy/sell guns, but too many join just for that.
And they use PMs to try to circumvent the 50 post rule.
Some more info in this thread
https://www.indianagunowners.com/forums/break-room/408311-info-new-guys-looking-ads-section.html