You couldn’t rent first-run movies through Amazon before COVID-19 shut theaters.
Amazon would rent movies that had already hit the rental market for between $2.99 and $5.99, depending on how recently the movie was in theaters. I just checked, Amazon still charges $5.99 for Parasite, the exact same amount I paid when I rented it through them 6 weeks ago.
With theaters closing, the big studios still wanted a way to show their movies, so they brought them to the rental market sooner and they’re charging substantially more because the movie should still be in theaters and charged $19.99 to stay closer to movie ticket prices. (They may have some data that says something like 1.8 people on average watch a movie streamed through Amazon, and that’s why they charge more than the price of 1 ticket, but I don’t know for sure.)