The thing is if you know you will never need to title it for street plates or get a loan on it then most times people will not transfer a title, cause that means they will have to pay sales tax on it at the transfer. So often the titles get lost but some folks dont worry about it if they never need to plate it or barrow against it.
In Texas as of 2006 they value it per the NADA value for tax amounts. So a lot of people dont want to pay the extra for a dirtbike so they dont bother with a title.

But you dont have a choice when buying from a dealer, in most states you get a title when new (it may say "Off Road Use Only" for dirt models) but you get a title.
A few (very few) states will issue the an MSO (manufacture statment of origin) that is like a birth certificate from the manufacture. Maybe OK is one of those states? In those states you may not have to pay sales taxes when buying it if it is intended for off road only.
I once got a bike with only an MSO from Mississippi and it was a real pain to transfer that into a title, but they (the state DOT office) made sure they got paid the tax portion!

It's kind of a "catch 22" most of us would want the title but dont want to pay the sales tax to transfer ownership. At least on a used dirtbike.