Alt-tab for Mac

Command-Tab (⌘tab) for application switching.

Control-down arrow (⌃⬇︎) for application windows.

Why? Because they are different operating systems with different developers and designers who make similar things differently.

There are thousands of big and small differences between Windows and macOS. If you want macOS to operate exactly as Windows does, there'd be little point using macOS.

You'll find some differences are better for you personally, and some are not. Everybody is different, and has different priorities and preferences for how they like things to operate.

Often there are alternative methods or third-party utilities to add Windows-like functionality to a Mac - in the same way that there are utilities for Windows that add Mac-like functions.

Neither operating system works exactly how each individual user wants. You have to look at the bigger picture and choose the one that suits your preferences and priorities most often.