From PR 1308:

Summary

Remove all uses of reflect.Value.MethodByName since it prevents dead code elimination. This causes binaries built with the mongodb/mongo-go-driver library to be larger than they need to be.

All occurrences of reflect.Value.MethodByName are replaced with type assertions.

Source: https://go.dev/src/cmd/link/internal/ld/deadcode.go#L293

Background & Motivation

Reduce the size of binaries built with mongodb/mongo-go-driver.

From the linked deadcode.go code:

// The third case is handled by looking to see if any of:
//   - reflect.Value.Method or MethodByName is reachable
//   - reflect.Type.Method or MethodByName is called (through the
//     REFLECTMETHOD attribute marked by the compiler).
//
// If any of these happen, all bets are off and all exported methods
// of reachable types are marked reachable.

TLDR: If your code calls reflect.Type.Method or reflect.Type.MethodByName, the linker can't prune any unreachable code and your compiled binary may be inflated. Never use those functions if you can avoid it.