Why don't set a Trial Period when it comes to publishing my app

I'm sorry if i have elected the wrong Forum for this Kind of question..

Hello communitiy,

i thought about a new way to publish my app. I don't want to make it completely free and i don't want to Show ads. That's because i spent hours to get the best Performance in start up and executing. (Ads take Long to be displayed, because they Need to be downloaded. And of course, they are annoying).

I also want to get free of the idea to make only half of the app accessible regarding to the free Version of my app..

So i thought about, why not give every user the full Version of my app for free and set a timer so that they can try and examine the app for a specific time. If they like it, they can buy, if not the app get unaccessible.

The simple code would look like this:

PackageInfo myPackageInfo = getPackageManager().getPackageInfo("com.android.myPackage", null);
myPackageInfo.firstInstallTime;

and over the System time i can determine if the Trial period is expired.

For sure, the user can now set back his System Time which will cause bad effects on his Alarm clock. Or he can simply uninstall the app and download it again. But in my app you can make many customized configurations, which i think will prevent most users from uninstall and reinstall the app again every 6 hours for example( if i set the Trial period to 6 hours). I think this Kind of Trial period is not unkown, many Softwares for Windows(i'm only up to date at Windows) have Trial Periods. And they use the Trial-period-idea since years.

So,

The Question

Why do i never see a app which got a Trial period. Is this prevented by Google. Or is it just a psychological Point regarding to applications.