No, the question is "why do we even need invasive anti-cheating mechanisms" with the proposed alternative being to simply ban people (of course, bans are still used even with anti-cheat).
Yes, banning a license key (assuming you have an unforgeable proof of license key ownership) is more potent than banning an IP address or email address. There are cross-game mechanisms like Steam VAC bans and Xbox Live account bans which are pretty potent too.
But they can still be evaded. Besides many cheaters simply having the money to buy new things, they can also get them from sites that trade in stolen license keys and accounts.