From a mechanical perspective, legendary actions are designed so boss monsters can stand a chance against entire parties. Otherwise, it becomes too easy for groups withe four actions to counter their one. In that respect, acting every round makes sense.
For larger or smaller groups this could be adjusted. Or not...
This logic has never made sense to me in 5E. There is no practical difference between a monster with a multiattack claw/claw/claw/bite and a monster with a bite attack and three legendary claw actions. It's not the raw number of actions that matters; it's the product of DPR and HP on each side. (Cf. Lanchester's Square Law.) Plus of course other considerations like stealth, mobility, magic, etc. The DMG math acknowledges this, which is precisely why it treats Legendary Actions just like any other action: you just roll it into the DPR.
In practical terms, I've had zero problems terrifying players with solo creatures, when terror is appropriate. A single tough creature is less vulnerable to AoE than the equivalent adjusted XP of smaller creatures would be; it has an easier time coordinating its actions with itself than a whole group would with each other (especially important for hit-and-run scenarios); it doesn't degrade in effectiveness until it's completely dead; unlike a group, it can restore itself to full effectiveness just by fleeing and resting/spending HD (whereas a group will leave behind casualties who don't come back to life after an hour of rest); it has higher saving throws and more effective skills. It typically has much less HP than a whole equivalent mob of lesser creatures would have, and it probably does less damage, but those factors simply compensate for its increased effectiveness in other ways.
The real "problem" with solo creatures in 5E is the expectations game: you have to realize that 5E encounters are generally designed to be easy, and 5E monsters are generally inefficiently designed compared to e.g. the Tome of Beasts or the DMG guidelines. But a properly-played Strahd or a proactively spellcasting dragon will rip the face off parties who would handily beat an equivalent XP value of orcs or wraiths or whatnot.
Solo monsters are not weaker than mobs. They are just different.