Because it doesn't pass very simple logical tests.
1) Harbaugh earns over $7MM, and is signed through next season. (apparently you thought he only earned $4MM and was only signed through this season, were that true, I would agree he is a possibility, but it's not). I don't know what website you reviewed, but we (collectively) discussed Harbaugh's numbers here last week. Texas is not going to pay its next coach $7MM, so he'd have to be willing to take a paycut.
2) Harbaugh has a record of 80-54 as HC of the Ravens, with a postseason record of 10-5. That is the 6th best record among active coaches with over 20 games as HC. Under Harbaugh, the Ravens have made the playoffs in 6 out of 9 seasons.
Far more likely than taking the Texas job is that the Ravens and Harbaugh will announce a contract extension this coming offseason. If he leaves Baltimore, it will be for another NFL franchise.