Contracts require (amongst other things) counter-parties who also sign and the exchange of goods or services between the parties ("consideration"), neither of which unilaterally signing some random document has.

In law, it is very difficult for individuals to bind their future selves. You generally need another party to create obligations that they can enforce against you. So for example counter-parties to a contract, trustees and beneficiaries, employers and employees, fiduciary agents and their clients etc etc. All of these require distinct legal persons to work, if you you combine them into a single legal person the obligations evaporate.

Because when it's you vs you the law pretty sensibly throws its hands in the air and says "well what the hell do you want the courts to do? Hop in a time machine and drag past you into court?"

IANAL, TINLA.