This is because the results are not shown as the total number of uses, but as the percent of all words used. Fewer works were published and exist in the corpus Google searches in the 19th century than in the 20th century, so a single mention has much greater effect. The word "the" has undeniably appeared many more times in the past 50 years than from1800-1850, but we see the same pattern in its ngram. For out-of-fashion words like condign and rapine, this effect is more pronounced.

blemish ... betroth ... rapine ... ravish

Look at words like obese, hyperbolic, and synergy. All of those words have existed since at least the 17th century and follow the opposite pattern - they've become much more widely used over the last 200 years.

Why do most of the word definitions I search on Google have a historical usage curve like this?

I find the same thing, but it may simply be that I'm looking up words that I don't know because they're less commonly used these days.

It seems like your question raises the more general one of how English speaking people's vocabulary has changed, but I suggest that you'll get better answers on /r/linguistics.

The words you have chosen are just words that are falling out of use. "Rapine" and "ravish" have been replaced with "loot" and "rape". Betrothal is a cultural institution that is disappearing. "Blemish" is being replaced by "flaw" and similar words.