> Is this really why the rule was made? Did they really have safety of driving in mind or is this just in the interest of the profits of airlines?

Yes. There are multiple public documents showing this, ex:

> The FAA has determined it is not appropriate to mandate the use of CRSs in aircraft now. We remain concerned that if we require children under 2 years old to be in an approved restraint system (which requires a passenger seat), affected operators might find it necessary to charge a fare for transporting these children. (Currently most, if not all, operators do not charge a fare for children under 2 years old who are held in an adult's lap.) In turn, for economic reasons some adults might decide to drive in automobiles to their destinations rather than fly. The FAA is concerned because automobile injury and fatality rates are higher than aircraft injury and fatality rates. As a result, there would be a net increase in transportation injuries and fatalities as families opt, for economic reasons, to drive rather than fly to their destinations.

https://www.federalregister.gov/documents/2005/08/26/05-1678...

> * Subsidize the 2nd seat for parents with children from tax dollars

That's not something the FAA or DOT can do.

> * Criminalize many intentional driving violations

That's not something the FAA or DOT can do.

> * Build high speed rail

That's not something the FAA or DOT (unilaterally) can do.