I think it is fairly simple: as long as screen resolutions increase, the default DPI(100%) will need to increase as well, to prevent "too small" UI.
Windows 7 released in 2009 and at that time, full HD monitors were not a thing, I guess.
Have a look at this post on MSDN Blogs but I'll quote the important part:
A change was made for Windows 7, to set the screen resolution to the monitor's native resolution and if the screen is over a certain resolution to default the font size 120 DPI (dots per inch). This is approximately 125% of normal 96 DPI (100%). When the system defaults to 125% DPI it also adjusts the MS Sans Serif font to be bigger. If the user then decides to set the DPI back to 100%, then TrueType fonts, which are scalable, adjust as expected.
The author of the MSDN article also followed this:
I installed the Windows 7 Ultimate build, it did not have the video driver and so defaulted to a lower resolution at 100% DPI. Once I installed the video driver from Windows Update, I was able to get to the native resolution and remain at 100% DPI.
The issue is not an internal problem, it can occur on any Windows 7 system that has a high resolution monitor for which drivers can be found during the initial install and so default with 125% DPI.
Windows 8 and 10 followed the same perspective.
Hope this helps!