That isn't always the case.
Windows has many built in APIs that devs are practically required to use if they want any reasonable about of performance with their AV scanners.
Any AV you can name off the top of your head is going to be using these APIs. Effectively this makes all other AVs just a reskinned defender.
If they detect more, it's likely because they are using more of the enterprise features that defender has turned off by default or are only available with pro, enterprise, or Azure subscription.

Usually the reason these features are disabled, besides a paywall, is because they will restrict your ability to use your computer and slow it down not insignificantly. The common person usually doesn't need or want features like IDS/IPS, Heuristic based AV, restricted memory use, etc.

You have to understand that AV, generally speaking, is security theater. It really can only detect known threats reliably. It won't do hardly anything against any new threat unless it's poorly developed or uses binaries from another known threat. AV gets bypassed every day. It will never be a surefire thing.