Suppose a monster is chasing the party, who blast a 10' pit (DC 15 to jump) in it's path. Is there no less DM fiat when I just decide whether the monster makes it across instead of rolling? After all, I've decided how good a jumper the monster is.
This is a very good question because it illustrates when randomness is actually useful.
But before engaging that question, let me point out that by determining what the jump skill of the monster is, you have predetermined how useful the players' strategy is. So if this is a standard strategy of your players, but all your monsters have especially high jump skills, you are still blocking the players, whether you roll the actual skill check or not.
However, you still want to roll for two reasons
First, the randomness is visible to all players. As I have mentioned upthread, randomness adds fun to those in the game that observe it, here it is observed by all at the table, not just the GM.
Second, when jumping/failing to jump over a pit either outcome is equally likely to contribute to the game. As many in this thread have mentioned, rolling up too tough a random encounter requires heavy DM adjucation to not derail a game. And failing to roll any random encounter in a long time can make some trips feel really easy or boring.
So random encounter carry a higher risk to the game while serving as entertainment only for the GM. That does not mean they are necessarily bad, after all the GM needs entertainment too. But I would argue that random encounters are a
bug for inexperienced DMs that don't know how to modify random encounters on the fly, while being a feature for very experienced DMs that can do so without strain or fail.
Similarly, if there's a 10% chance to meet 1d4 trolls each day when crossing the Troll Mountains, why should I just decide that now's a good time for them to meet three?
It will make no difference to your players, unless you let them peek behind the curtain by letting them know that you roll for wandering monsters every day.
Often I don't actually decide even how a jumper the monster is or what the random encounter table looks like, because the DMG/MM/adventure tells me.
By that argument, I don't predetermine encounters in an adventure either, because the adventure tells me.