I'm glad you've asked! I actually mentioned this in a thread ages ago.
The issue with that sort of proposed solution is; it becomes a non-choice, because you just pick whatever you know/you think you're most likely going to see today. You might as well just have the bonus regardless.
And then also, the ability to pick something different detracts from the roleplay aspect, it's no longer a meaningful part of your character's backstory or identity. "My home town is raided by Fiends at the end of each season... But, we're exploring a Haunted Manor today, so my Favoured Enemy is Undead."
And worse, you're being forced to make a choice at the start of the adventuring day when you might have zero information about what you're actually going to fight, thus opening the door to picking wrong anyways. Just looking over my last adventure session, the players had three encounters, one with beasts, one with celestials, and one with fey. Based on what they knew about the scenario, I'm certain a Ranger player would have chosen fey, because they'd faced some already, but they'd also already faced dragons twice, beasts twice, and an encounter with monstrosities.
The problem is? The fey encounter was the easiest of the three and wouldn't have required a bonus. Now maybe it scouting ahead was a more viable strategy in D&D, you could do that before the long rest, but typically, that just puts you at risk of having to escape a solo encounter since stealth is unreliable.
Primeval Awareness isn't even all that helpful, since it requires you to still have a spell slot to use it, only has a 1 mile range, and tells you only if a creature type is present, not location or number- unless you're in one of your favored terrains, another unreliable feature, which at least ups the range significantly.
So you could use a first level slot, find out that there's a dragons within 1 mile, swap your FE, end up encountering a lone friendly Fairy Dragon and then find a cave with an underwater grotto with an Aboleth and a pack of Chuul inside.
Changing FE gives you more chances to get it right, so it's less unreliable, but it's still not reliable. Great for a ribbon feature, not great for a prime ability.