Infinite scroll has its uses, but a search result page is not one of them. Infinite scroll relies on discovery and exploration. This works well for when you’re not looking for anything in particular; when you’re sifting through a massive amount of data until you find something that hooks your interest. This format lends itself extremely well for social media with newsfeeds, or inspiration platforms like Pinterest.
You may measure the benefits of infinite scrolling with the example of a Facebook news feed. By unspoken agreement, users are aware that they won’t get to see everything on the feed, because the content is updated too frequently. With infinite scrolling, Facebook is doing it’s best to expose as much information as possible to the users and they are scanning and consuming this flow of information.
Search results are all about finding what you need.
Pagination is good when the user is searching for something in
particular within the list of results, not just scanning and consuming
the flow of information.
Google wants to show you relevant data. Pagination tells you where the search results are located (page) and in which order they match your criteria. This helps to estimate how much time it’ll take to find what you’re actually looking for and helps you find results back.
Google has experimented in the past with loading times. Turns out, even with a slightly longer delay in loading, people are more likely to abandon the search. Infinite scroll is rather heavy on performance, so this would multiply this abandoning rate.
More here: https://uxplanet.org/ux-infinite-scrolling-vs-pagination-1030d29376f1
There is no official statement regarding this, so no one can give you a 100% accurate answer.
One of the biggest reasons might be the ad placement of Google.
They earn money with every search, and since they use pagination they can even earn money multiple times with only one search, try it yourself.
For example if you search for “toaster” (in Germany), these are the 1st and 2nd page ads:
So in short, I think pagination fits Google’s business goals.
Since no one posted it yet, to me the main reason against infinite scrolling is shown in this XKCD comic:
The tool tip reads:
Maybe we should give up on the whole idea of a ‘back’ button. ‘Show me that thing I was looking at a moment ago’ might just be too complicated an idea for the modern web.
Here is the Explain XKCD page for the comic strip.
I just found the hidden comment above that already mentioned this XKCD comic strip.
To clarify, why I do think my answer is valid here is: To me, the equivalent of the above
“If I touch the wrong thing, I’ll lose my place and have to start over”
is when clicking a SERP entry, going to the destination URL and then clicking the Back button.
This would usually return me to some other location than the previous SERP infinite scroll position where I left.