1. If I haven't used the domain in 2 years, it's never going to happen so I will let it lapse
That in itself may not be enough, as you may become attached to some of the names, because they're just too good. So there is also rule 2:
2. Max 12 domains at any time (1 for each month), to keep costs reasonable.
If I'm at 12 domains, and I have a really good idea for another one... I have to weigh it against the existing domains and either not register it -- or immediately turn off auto renew for whichever other domain lost against my brilliant new idea.
Somebody else decides your car is perfect for them and is willing to buy it off you for far more than it has cost you.
At what price would you sell the car?
Even mass produced cars are rarely identical. From new, there are a huge number of ways cars can be optioned, and on the resale market there are myriad variables that can make a car more or less desirable to different buyers.
Importantly, unless you literally tell a given seller that you won't settle for a different make/model, they have to assume that you might, and thus in their minds, they're competing for you with a lot of other people. As a buyer, you'd have to work really hard to get an unreasonable quote.
The same applies to domains. Some people don’t care if they need to add an extra word, or pick a different extension. To other people it matters a lot.