Can someone please explain to me why this PC can barely run most modern games?

Solution
I'll give You plenty of reasons. First, and foremost is Your graphics card. It's office graphics card, good for movies [GT 730]. Second is Your CPU, which needs overclocking, or changing for something more solid, so it can feed some normal graphics card. Third is Your 1TB SATA drive which can store Store 666,000 photos, 285,000 songs or 526 hours of HD video and more, but cannot run a gaming system, since You should get solid state drive. Fourth is Your most likely cheap motherboard, which I cannot determine, because it's not specified, but I believe it's gonna be some "lite" motherboard. If You post me Your motherboard model [CPU-Z] I can advise You how to upgrade this "gaming disaster".
I'll give You plenty of reasons. First, and foremost is Your graphics card. It's office graphics card, good for movies [GT 730]. Second is Your CPU, which needs overclocking, or changing for something more solid, so it can feed some normal graphics card. Third is Your 1TB SATA drive which can store Store 666,000 photos, 285,000 songs or 526 hours of HD video and more, but cannot run a gaming system, since You should get solid state drive. Fourth is Your most likely cheap motherboard, which I cannot determine, because it's not specified, but I believe it's gonna be some "lite" motherboard. If You post me Your motherboard model [CPU-Z] I can advise You how to upgrade this "gaming disaster".
 
Solution