But ф is absolutely not the sound that "th" makes.
In fact, in Russian, at least, we don't have a "th" sound.
But this is better than using Д in place of "A".
It absolutely doesn't as pronounced now, yet Thomas is Фома, Theodor is Феодор, etc. Just like Hertz is Герц, even through Г and H are as far from each other as one can get.
Yeah, it is just фe most aesфetically pleasing letter for фis case.
Actually, for most of its existence, Cyrillic has had a θ (theta) like in Greek, used only in loanwords and pronounced either as ф (f) or т (t) because the th sound is not part of Slavic phonetics. θ was dropped fairly recently - in the 20th century.