The short answer is: if you did not sign an agreement that severed the mineral rights from the seller selling to you, AND that seller owns the mineral rights then yes, in Texas the mineral rights automatically "follow the land", ie, they convey to you.
Like most mentioned, in many cases the mineral rights were severed long ago by some previous owner. This could have been the farmers family that owned the farm long before it was developed, by the developer when he developed the community and sold to the builder, or by the builder himself. There could be many variations and even multiple heirships involved.
What I tell people about mineral rights when buying in large, developed subdivisions is that it's best that you are on a level playing field. For example, If the developer (or the farmer) severed the rights to the community as a whole, then no homeowner in the community or subdivision owns the rights - every one is on the same playing field. In older communities this may not necessarily apply and it may be impossible to even get on the field - because there isn't a level one.
There is no reason for a seller to not convey the rights on very small residential rights. The value of minerals are unknown, but typically will not be worth tons of money-the land area is simply too small. When it comes down to selling a small residential property vs. retaining the rights, sell the property and forget about the rights. I have a 100% capture rate for buyers on retaining the rights when the seller wanted to keep them (if they did indeed own them). The argument in favor of conveyance is pretty easily made if the seller wants to sell.
Some agents get confused when the sellers agent says "we can't transfer the rights - we have leased them already", Big deal - the lease should be and can be transferred to the new buyer, just like a tenant lease is.
That doesn't mean the seller gives up the bonus he received when he signed the lease (that is in the past and he doesn't owe that to a new buyer)- all it means is that any future royalties now belong to the new owner,
Mineral rights searches are not performed by title companies, they are performed by landmen. These are,more exhausive, way more expensive searches. A title company should be able to tell you whether or not there is a recorded lease on the land, but not necessarily who owns the rights.
Bottom line, if you didn't sign the severance agreement, and the guy that sold to you indeed DID own the rights, now they belong to you.