Dirt could be one of the issues behind it. Clean your PC if you haven't in months. I clean my PC regularly every month.
If your graphic card is old like 3 years or so then it could be that the thermal paste between the heatsink and GPU may have evaporated or diminished. If you know how to postmortem your computer then you can open the graphics card and put some high quality thermal paste, like from thermalright or something, & clean it properly. Temperatures will definitely go down considerably if that's the case.
And in addition you can use MSI Afterburner to manually increase the speed of cooling fan(s) when in gaming, so that they cool more and lower the temperatures.