The reason is they want to bundle stream quality to stream quantity to keep squeezing subscription money for features you don't use. I share my account because 4k streaming is locked behind their Premium tier, 4 simultaneous stream account. I pay double the price in order to get 4k streaming. If they don't want me to share my account, I'd be happy with a 4k quality, 1 stream quantity account, at a lower price. They don't want to do that. When they keep the pricing the same and now try to charge me for account sharing, I'll just cancel.

https://help.netflix.com/en/node/24926