"Legalese" can often be simplified, but concepts that are necessarily in law are often not widely known by lay people, and as such it's hard to avoid some terminology and reasoning that require a bit of legal training to understand.

But every subject and field has their own set of terminology. It's just like programming -- while we strive to make code easier to understand (eg. Python is better than assembler in this regard), there's still a necessary learning curve. Your question is almost like asking "why can't we just tell the computer what we want to do in plain English?"

Sometimes the legalese is actually comprehensible if you give it a bit of patience. Often though, programmers like to make (wrong) assumptions about how the words are to be interpreted though, and that's where most people trip up.

Without a common language with exact meaning for phrases that are accepted by both parties contracts would be impossible to enforce and become useless.