Understandable but hardly justifiable when voting machines first were implemented, but the provider has had many years to improve their product. That they don't suggests they are paid regardless of quality, or it is working as intended.
I think you may be overestimating how often these machines get updated. I don't know about the internals, but as a long time TX resident I can say that the machines I saw last time I voted (2016) didn't look much if any different than the machines I saw at least 10 years ago, and probably longer. Also, there's got to be some amount of overhead related to certification and training for each new design, so the state has plenty of incentive to keep using the same machines.
