The signaling for phone wire is less demanding, but ethernet is a digital protocol. That digital protocol will resist noise. Also CAT5 cable has the twisted pair cabling which negates the need for shielding.
Personally, I don't know enough about longer lengths of phone cable. I took an undergrad class in networking basics, but I don't know the signal loss associated with types of cables. If the signal loss is small enough for 200 ft of phone cable, then you would be correct that phone cable is the superior solution.
I just had another thought. How would one prevent critters from biting into the cable? 200 ft is a long enough length that it becomes very concerning how one would protect from vermin. And if the cable does get bitten through, how would you splice the broken cable such that it is weatherproof? Again, I'm pleading ignorance. I simply don't have the experience in laying down cable.