Supply and Demand 101
The math is simple: There are only 17,576 possible 3-letter .com domains. There are only 456,976 4-letter .coms. Every single one has been registered since the 1990s. To get one today, you have to buy it from someone else.
The "Trust Tax"
When a user sees a long, hyphenated, or obscure domain extension, their brain unconsciously signals "risk". A premium .com signals "established". You pay for the domain once, but you reap the benefit of that trust signal every single day in higher conversion rates.
Marketing Efficiency
Imagine two billboards. One says "Visit Super-Fast-Delivery-Service.net". The other says "Go.com". Which one do you remember? A short domain keeps your Customer Acquisition Cost (CAC) low because your direct traffic (people typing your name) is high.
The Asset Value
A premium domain is liquid. If your startup fails (statistically probable), your code is worthless, your office lease is a liability, but your premium domain is an asset you can sell to recover capital. It is a store of value.