Why does malware spread fast? It spreads fast because of two things:

The Internet is a semi-monoculture when it comes to software versions: this is because of practicalities: it is easier to hire somebody to work on popular software, it is also easier to build stuff using popular software.

There Internet scales:the Internet has connected everyone. All you have to do is compromise a small percentage of them.

Bugs: there are about 15-50 errors per 1000 lines of code. Cutting secure code is probably the best thing to do to keep the Internet safe.