Google uses Arial in the font stack across all of their pages, for consistency. I assume the following rationale is at least mostly correct.
In case a computer doesn't have that as the default for some reason, they don't want their page to look any different, if it is possible for the computer to display it correctly, because if it doesn't, it affects their brand image.
Imagine if you did a google search and everything looked just a little bit off (and you weren't a developer so you didn't know why): you might think someone was hacking google and stealing your information or changing your results. Consistency builds trust, and this is important to them.
In case a computer doesn't have that as the default for some reason, they don't want their page to look any different, if it is possible for the computer to display it correctly, because if it doesn't, it affects their brand image.
Imagine if you did a google search and everything looked just a little bit off (and you weren't a developer so you didn't know why): you might think someone was hacking google and stealing your information or changing your results. Consistency builds trust, and this is important to them.