Most sellers are retaining their mineral rights if they own them... as they should. When my buyers object to this I ask them 2 questions: first, " Wouldn't you want to retain the mineral rights if YOU were selling your property?" and "What if we discover that the seller doesn't even have mineral rights?... would you change your mind about wanting the property?"
On a typical quarter-acre lot in Fort Worth the up-front bonus has gotten up to around $4,000, with monthly royalties coming in between $50 and $150. If you were to invest that income over the expected 30-year production term you would wind up with well over $100,000. So, you can easily understand why people are generally not just throwing this asset in with the home purchase.
I also have a contract with a new homebuilder in Collin county and they used a "mineral rights addendum" where they will not transfer because they didn't get them from the developer... weird huh?