What I do:
1. Click the three dots in the upper right hand corner of the web browser
2. More tools -> create shortcut (pc)/create shortcut (mobile)
3. "Open as window)
On Chromebook, this pins it to the apps bar, mobile it pins it to the home screen and I have no idea for other devices
There is an app on the Google Play Store now. Similar to the website, but a little easier to manage on your phone and there's no address bar in the way.
Why? It would mean they have to maintain another code base and paying for distribution services like App Store and Google Play. This would mean more costs which means requiring a subscription to use it and/or more advertising.
How would you expect imgflip to exist? Even if the creators and developers worked totally for free, it cost money to host the site and pay for things like ssl certificates, domain names, etc.