Etymonline on "-able" doesn't expound the origin of "requiring".
common termination and word-forming element of English adjectives (typically based on verbs) and generally adding a notion of "capable of; allowed; worthy of; requiring; to be ______ed," sometimes "full of, causing," from French -able and directly from Latin -abilis. It is properly -ble, from Latin -bilis (the vowel being generally from the stem ending of the verb being suffixed), and it represents PIE *-tro, a suffix used to form nouns of instrument, cognate with the second syllables of English rudder and saddle (n.).
For example, "payABLE" literally means ABLE to pay. Ability differs from requirement. How did "payable" semantically shift to meaning 1 below?
1. (of money) required to be paid; due.
- Able to be paid.
payables Debts owed by a business; liabilities.