Why are there so many apps for Android but they're not available for Apple?

Because of several reasons:
1:Apple keeps tight reign on apps. Only vetted, apple approved apps get on their store. Android is much more open for developers. This is the main reason. The drawback is a lot of android apps are complete junk and some are of illicit intent.
2. Android development is free and this makes it more open to hobby developers.
3. Android apps are java based and google has a rich set of tools readily available. IOS apps are programmed in objective C which is a great language but not so easy for a novice. It can get hairy with all of the pointers that you don’t have to mess with in Java.
Android gives you flexibility, features and is more open and IOS gives you stable, reliable apps but is somewhat sandboxed. Both approaches have their merits.

Developers are often independents who develop apps for one or the other platform. Software companies usually have separate teams that develop apps for each platform at different rates. Apple used to have many more apps than Android so it varies from time to time. Both of my sons work in mobile development and the younger one got his first position because of the success of a music app he created for Apple as an independent.

The number of people using Android vs. iOS.

Apple was founded in the 70’s, yet went almost nowhere as a company for decades- they offered really low rates to schools, and survived mainly on selling systems to schools and other public institutions, while PC’s- primarily windows-based machines- thrived and took over the majority of the market share.
This was due to Apple’s refusal to release the source code to the aftermarket- they are the ultimate control freaks. You won’t find hackers running Apple hardware very often, if ever.
IBM clones are like hotrods- users can modify them, speed them up, (overclock), change hardware components, modify the registry, change OS’s, etc. Apple is tight fisted and paranoid about anyone other than Apple even opening an Apple machine, much less modifying the software or writing programs to interface with their software.
The majority of users who like to modify/upgrade/write programs, tend to be Android users, because of these facts. Apple products tend to be purchased by people who are more concerned with what color their computer or phone is, than how much RAM it has or whether you can gain root access. These are typically also not the people developing new apps.

Apple products aren’t bad- there are a few advantages to Apple’s attitudes- most notably, the system was not quite as at risk from spammers and viruses, although this is changing rapidly now.
Personally, I don’t care for Apple’s bloated prices or their “You’ll take what we give you and like it!” attitude, but, to each his own!