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The binaries are signed with X.509

Why should they be?

It makes it much easier to MITM and let the user download a drive-by infected binary.

How is it possible in this day that Microsoft is not able to provide a decent SSH client for Windows?

They do... both native and via subsystem for linux.

If that's so why are we still using PuTYY / KiTTY (serious question)?

Well, I would guess first and foremost: why change? People that already use putty probably have no desire to switch just to switch. If the alternative isn't SIGNIFICANTLY better, most people aren't going to switch just to switch.

Second: not everyone is on Windows 10 which is a requirement for the Linux subsystem.

I guess last but not least: in both cases they require installing software. Some corporate users have no control over their own laptops. Putty has a portable version that you don't have to install anything to use.

Perhaps you should consider doing your research on this subject before writing such a contentious post?