If "contractual capacity" means that you have a legally robust understanding of the meaning of all of the clauses in a contract, then I'd posit that maybe 1% of the population have that, even for common consumer contracts such as with Telcos.
I'd guess that only a small minority of the population have a sound idea of what arbitration or indemnification are for example.
"Contractual capacity" means you have the capacity to enter legally-binding contracts. It is generally understood that if you willingly enter into a contract that you agreed to the terms. Not willingly entering the contract is signing under duress and would result in a voided contract. Not understanding the terms is not a valid defense, unless you were not capable of understanding them, in which case you would be ruled contractually incapable and any contracts voided. If you were capable of understanding the terms and simply chose not to before signing, that is on you.
I'd guess that only a small minority of the population have a sound idea of what arbitration or indemnification are for example.