The thinking is probably that if you allow plurals and certain other verb forms to any significant degree you probably end up with a disproportionate number of trailing "S" letters--which would be less interesting.

The combination of five letters, the rules, and the number of guesses seem to calibrate the difficulty pretty well for me. Given enough time, I could probably solve most words eventually with just my brain. (In practice, I sometimes go to a dictionary program that's not wordle specific to tickle my brain once I know what I'm probably looking for.

The reason for the restriction is that you make a near endless amount of words by allowing plural, past, future, genitive (at least in Danish, that doesn't take apostrophe in the genitive) et cetera. I'm unsure how I would restrict adjectives/adverbs, but the idea is that the words should exist as main entries in a dictionary.

Here's where I come from:

Decades ago I learned to play a word game which was more like MasterMind than Wordle, and it could be played with just pen and paper (later I have played it over sms, and over gratis bank transfers):

Each participant writes down a secret 5-letter word (unique letters, singular nouns, no verbs) and take turns to guess the oppponent's word. The only clue you get for each guess, is how many correct letters your guess has. You do not get to know which letters are correct, if any, nor if they are correctly placed. It's a bit more challenging than Wordle, something that justifies the restrictions, but even with Wordle I think it's reasonable to use just the “root dictionary words”, if I may call them so.

I know the original Wordle also don't use 5 unique letters, that's why I wrote “all wordle authors” so you wouldn't feel singled out.

I chose words for a Danish wordle, and distilled 64k dictionary words down to around 3k.