Besides, of course the high frequency of security updates is highly welcome. But on the other hand one could ask why so many of them are actually needed to be released in the first place...
Because those kinds of bugs just happen; that's 30 years of software/systems dev talking.
There millions of lines of codes involved here, countless possible paths and user entries that interact together. With time, coding standards/design and even the languages themselves and IDE tools have mitigated most of the type of security bugs that used to plague software, but some more subtle ones still slip through.
A system that evolves fast has more of them since your changing more code, more interfaces and adding/removing/changing things all over the system.
The more services/means of access a system provides, the more potential entry points it provides.
For example, If there were user accounts, that would be one more thing to worry about...
Hardened systems have few services, access points, users, all of them monitored from another system which doesn't depend on the first.
Hardened systems tend to have few features/services and honed them to death.
Most people would complain if IOS became like that.
Unlike those systems IOS (and Android) must be hard to compromise WHILE most of the opposite of those hardened systems. This exposes a lot more code to outside forces.
Only in the last decade, with much of our lives becoming digital, has security become less than an afterthought on digital platforms. There has been an obvious learning curve for the entire industry. Things are improving though; nobody would trust all their info to a phone with XP level security!