Explanation: in order to have defences (mostly, but not wholly, talking hit points here, and then save resistances) to last long enough to even register as a "solo fight" you need to pick a Legendary so many CRs above the APL that its offense will threaten to oneshot a player character, any player character. This in turn cheapens the value of being a "sturdy" party member.
It cheapens the value versus the super deadly legendary beast from hell capable of fighting entire groups by itself.
Naturally "tanking" is harder in that instance, this beast has slain hundreds!
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However, if using the "boss" fight at the end of the adventure segment theory, yeah that would get old quick, and is a valid concern.