How do you think it ought to read instead?
I'm sure there's a reason, but I always thought it strange that when the subject in singular, the verb often gets an 's' ending (mirroring the plural ending for most nouns) but when the subject is plural (often ending in an 's') the verb normally doesn't get an 's' ending.
The frogs play in water. The frog plays in water. Why does the 's' effectively swap locations?
Why not, "the frogs plays in water." Or "the frog play in water"?
Or why not drops the singular/plural distinction from verbs altogether?
I am pretty sure the etymology of the plural ending -s and the third person present verb ending -s (which used to be -eth in pre-modern forms of English) are unrelated, and they just sound the same by coincidence.
English evolved a 3rd person singular verb ending. It randomly happened to be "s". It also evolved a plural noun ending. That too randomly happened to be "s". There's nothing deeper going on than that.
And I think that's strange. That's it.
You could also go Indonesian: repeat a noun for plural. "Frog-frog play in water!"
Why not use different morphology for these different grammatical concepts? The language would be cleaner and more consistent. My theory is that, basically, it's hard to learn to pronounce many different sounds, but it's easy for our brains to sort through the ambiguity that the morphological overlap produces.
The only inflection English retains in the present indicative is the -s suffix for 3rd person singular (frog). Third person plural (frogs) does not have an -s.
The -s suffix for noun plurals, and for possessives (vestigial genitive case), add exceptions that make learning English a little harder.
Edit: grammar :)
A publication may one day be able to accumulate those who care the most as their customer base, and thereby assure the production of grammatically correct ephemera, but for now, other business interests prevail.