I could go into detail how being able to have a dozen tabs open almost always gives a better result than simply picking the first flight on google flights. But let's assume there's no difference:

Do you really want to use a phone's on-screen keyboard to type in your family's passport details, address, then credit card numbers, then review all of this to ensure your $2000 purchase doesn't have any typos or mistakes? If you have the choice to use a real computer for this, then why not? It's not like booking a big trip is something you do while sitting on a bus.

Then of course there's accommodation, itineraries, visas, trip research...