Why is my big chainring is so scratched form the inside?
Some damage is to be expected caused by upshifts, that said, anecdotally speaking (i.e., based on personal experience), that amount of damage looks excessive.  

Speculation

I wonder if the front derailleur was misadjusted so that it could be in the upper position, but not far enough for the chain to properly catch and upshift.  If the derailleur was left in the upper position the chain could partially catch then drop back down to the lower ring.  If ignored and ridden in that position over many, many miles the chain would continual catch and drop, wearing the inner side of the ring. 

In terms of upshift functionality, all that really matters is whether the ramps and pins remain functional.  That is hard to assess from the photo (although I see some intact pins).  



Next Steps

The best thing to do is make sure that the front derailleur is adjusted properly.  If the upshifts are still acceptable, then don't worry about the damage as it would be largely superficial in this case (i.e., the pins and ramps can still do their job). If you cannot get it to upshift reasonably well then you should look at replacing the affected ring(s).

Finally, with that much wear it is also worth taking a look at the teeth profile the teeth themselves are excessively worn, such that a new chain may skip.  This would be a different problem than the inner surface wear discussed here. 

See: Does this chainring need to be replaced?