Like almost any other industry, VPN providers want to keep hold of their customers for as long as possible, out of reach from their competitors.
Therefore, long-term plans are made incredibly appealing, and short-term plans are expensive. It could also be argued that having an expensive short plan makes a cheaper long-term plan look more appealing – but that's a different story...
The long and short of it is that cheap monthly VPN plans are hard to find, but some providers like Atlas VPN deliver genuinely good-value plans that you don't have to commit to for years to get.