Mostly, I think, legalese is a collection of superstitous incantations built up over millenia, which lawyers use because "you need to say it exactly like this or vague, bad stuff will happen." Except sometimes the superstitions are right.
I also think lawyers don't find it worth optimising the language much, because the target audience is other lawyers who know all the incantations anyway. The target audience is lawyers instead of laypeople, because the language isn't what matters in the first place. What matters is all the relevant case law, which can vary so much from the "obvious" or even "right" interpretaion of the written language, that a layperson not using a lawyer for all but the most trivial legal needs is a recipe for disaster.
I also think lawyers don't find it worth optimising the language much, because the target audience is other lawyers who know all the incantations anyway. The target audience is lawyers instead of laypeople, because the language isn't what matters in the first place. What matters is all the relevant case law, which can vary so much from the "obvious" or even "right" interpretaion of the written language, that a layperson not using a lawyer for all but the most trivial legal needs is a recipe for disaster.