Something in the rules certainly does prevent that: "you aid a friendly creature in attacking a creature within 5 feet of you." If the familiar is more than 5' away, it's no longer Helping.
That's only one interpretation of those words. Many people interpret "you aid a friendly creature in attacking a creature within 5 feet of you" to mean that when the Help action is started, the helping creature has to be within 5 feet. Nothing explicitly in those words indicate that he has to stay there after performing the help action, or that the help action is a round long activity that ends if the helper is incapacitated.
Go read this thread.
Movement and Help
If you want to think that your interpretation is the only one, go ahead. It doesn't make it true.
So if the familiar is an owl you would rule that Help doesn't linger because owls don't provoke when they move away, but Help from other familiars does linger because they provoke if they move away (regardless of whether they actually have or not)???
If someone does not provoke with an Owl familiar over some other familiar, great. But, lingering works the same regardless. Or at least at my table.
So according to you, a familiar can come in and Help and move away, and that Help would linger but still only come into effect for the ally specifically designated by the familiar even though it is nowhere near the opponent at the time that any of the allies acts?
Yup. We are talking about a fantasy game. RAW (based on my interpretation) states that it works that way and it is not important enough to worry about or house rule, so that's what happens.
I can easily add color commentary to support this. "The weasel familiar barrels past the orc, partially climbing up his pant leg before moving on to pull the orc's attention away from the wizard. The orc swings at the weasel (rolls dice), misses, and keeps track of it, ignoring the wizard, but still focusing on the fighter.".
Course, there are other rules one could question, so why bother?
If this type of thing bothers you at your table, of course you are capable of interpreting and adjudicating it any way you like.
Personally, the action is called Help, not Distract. So, I am ok with Help helping one specific PC.
How does the familiar affect the Help after it has moved away from the opponent in order to pinpoint a specific ally? According to you: a familiar can move in, Help, and fly out; the opponent can then act (or react) killing the familiar; Azrolphus (PC wizard 1) can then cast Fire Bolt at the opponent without advantage because he wasn't the designated recipient of the Help; but Bramafax (PC wizard 2) does get advantage when he casts the same spell at the opponent because he was the designated 'helpee' - and all of this after the familiar is dead. How does that work?
Who cares? I play the game to have fun, not to worry about making sure every niggly little rule makes 100% sense 100% of the time.