Computer question - in details?

How come when I upgraded my memory (RAM) on my laptop, I had to reload the OS. For some reason it wouldn't allow the hardware change, btw this is a Windows 10 laptop. It didn't like the upgrade even though I installed the same speed RAM and same double data rate (ddr3). Both old RAM and the new RAM were pc12800. So I don't get why I had to reload the entire operating system in order for it to 𝗼𝗽𝗲𝗿𝗮𝘁𝗲 Please can someone explain why it didn't work plug n play? I didn't have that much data so I wasn't really worried about that. I save most my stuff in the cloud anyhow. But it was a waste of time since the OS setup usually takes an hour to an hour and a half for it to factory reset and rollover back to Windows 10 (in tact). @master of puppets the old RAM was completely replaced. It was no longer in either dimm slot, it was brand new RAM in both slots. I don't know how you get through the installation in 25-30 minutes. You must have like a quad core or octa core cpu. Mine is only dual core and takes roughly an hour. As for the RAM upgrade it wouldn't boot past post into the operating system. I tried running system repair but it didn't work either. So I had to reload the OS.