You are wildly exaggerating. I have been to Peru where I had numerous ceviches served in the traditional way(ie fully raw and served with raw, freshly-squeezed lemon-juice), with no extra corn or additives or whatever.Never once was I served with extra corn or sweet potatoes, or indeed anything else.
I was not referring to sushi restaurants, but to genuine sashimi restaurants. Sashimi restaurants do also provide some cooked dishes, but their sashimi dishes are always 100% raw. My only dislike is that many sashimi restaurants serve their raw fish in nouvelle-cuisine-style, with tiny amounts. I did once encounter an exception where I got served thick chunks of raw mackerel and raw swordfish by a Japanese restaurant near King's Cross in London, but, sadly, they did not remain there long.