When you search for an app’s specific name, you likely already know what you want to get, and you probably just want a quick way to install it without going through any hassles. However, if you have a more generic term or if you’ve confused one app for another, it might make sense to first take a proper look at the detailed listing in order to avoid installing the wrong app or, worse, a phishing or scamming app (some will always slip through Google’s malware and abuse scanners). In those cases, Google likely wants to enforce this extra tap to make sure to at least give you the chance to read up on the app, its description, and its reviews before you tap that install button.
I guess it makes sense. Aurora Store (a FOSS frontend for the Play Store) doesn’t have install buttons in the search results at all, and F-Droid only has install buttons in the search results for apps without “anti-features” (i.e., potentially worrisome privacy practices, etc.).