Because they are apps. They have the same bundle id as game, so when they extract all assets inside and install game it will just replace installer on your phone and you can delete installer apk files. If it says it’s incompatible it’s one of the two - either you have new android and it complains about incompatibility due to 32 bit but you can ignore and still run them OR your phone doesn’t support 32 bit at all, then you’re out of luck, there are some ways to run 32 bits apps but they are complicated and don’t work on every phone. Eventually there will be 64 bit build but likely not this year.
Just wanted to give you props for this. Not sure if this was your idea or if you simply became aware of the concept and implemented it, but this is such a clever way to go about installing a large game like this on Android.
In case anyone reading this doesn't get what I'm talking about:
The installer is split into five APKs. Each APK contains the same (of at least, a very similar) binary/executable, and uses the same internal ID. So when you install the first APK, it installs an Out of Touch "installer" app, which then extracts ~1.5GB of the game's files to an arbitrary location in your file system. Once done, you then install the second APK, which overwrites (or, technically, "updates") the previous installer. When you open the installer app again, it extracts the second ~1.5GB of the game data. Rinse and repeat until all five (at the time of writing) installers have been run. At this point, it then installs the actual Out of Touch game APK, which it extracted from the 5 installers, which overwrites itself ("updates" itself), replacing the installer with the actual game's binary itself.
There's a ton of benefits from doing the this way, but one of the biggest benefits is that it can guarantee that none of the files are corrupted, and it saves the player the hassle of needing to extract a large (or many smaller) archive (.zip, .7z, .Rae, .tar.gz, etc.) into a specific folder specified by the game, which is how many other VNs of this size handle it on Android.
There are, however, security issues with this method, though one of these issues exists even with the games that use manual archive extraction. Some Android devices won't even allow you to run apps that use the older API version, which is necessary for an app to be able to access arbitrary locations in your device's storage (and this is likely to become more and more common with newer devices). More importantly, though, apps using this older API do present a significant security risk. This security risk is only made more significant by permitting the Out of Touch app to install APKs itself; the result is an app with complete access to your device's file system that also has the ability to install other apps. That said, that isn't really that big of a deal, since the user still has to accept the installation of a given app (as in, installing an app can't be completely automated). Personally, I trust that these devs aren't about to go doing anything shady.
In any case, this is a super clever way of handling this problem, and regardless of whether you came up with the idea or not, I just wanted to compliment you on your ingenuity! Super neat.