Well, first of all the obvious answer you already gave in the comments:
According to DMG guidelines, no, the probability of damage does not change the CR of a monster, but only the average.
Now for more things based on the experience.
In fact, the more attacks you have, the more gaussian the probability is, not uniform – a uniform distribution is in fact quite the opposite, one that has the same probability of causing any damage. What you probably want to say is that the distribution has a low variance.
In addition, the PMF (Probability Mass Function) of the damage depends on the damage dice. For example, an attack that processes 10d4 has a variance of less than five attacks that deal with 1d8. I guess for some reason you're using average damage instead of fixing it.
Ok, so, about your question. Fewer attacks does not make the fight easier or harder, it makes fighting more dynamic. Yes, there is a higher probability that you will inflict a maximum of damage, but also a greater probability that you inflict damage. no damagewhich would be an incredibly easy fight. This means that concentrating the damage on an attack makes the good rolls really good and the bad rolls really bad.
In D & D, and especially for the 5th, higher variance compromises low-level parties and is indifferent to higher levels. As you mentioned, in the early levels (CR2 for example), a creature inflicting 60 damage in 3 rounds could potentially kill a party member or force him or her to spend a huge amount of resources. On the other hand, since the damage does not increase as fast as the PC's HP and the party has more recovery resources at higher levels, three sets of striking damage are not required. this threatening to a 10 + level party.
For the very weak CR (1-), the monsters do not undergo multiple attack because their number of damages is already weak (1d8 or less).
For higher CRs, a good reason for a multiple attack is precisely to provide a lower variance in the attacks. It would not be very fun for me, as a DM, if my CR 10 creature did one thing in its entirety and that it failed, doing practically nothing. On the other hand, "the dragon tries to bite you and misses, but then he quickly uses his claws while you still dodge the bite and you get" at least gives me something to tell.
So, if you're looking for a basic rule, mine is: Swingy dice are bad. Saving or sucking features are not good when put against PCs, because a "suck" for a PC can be really, really bad and not fun for this player. To learn more about why I think swingy dice are bad, read about Goblin Dice.