I've asked this question in Quora and the answers I got were:

First answer:

Using "more" and "less" helps maintain clarity and consistency in comparative forms. It provides a straightforward and predictable way to form comparatives and superlatives without relying on irregular or unpredictable suffixes.

Second answer:

Longer adjectives often have complex or multisyllabic structures, making it more challenging to add suffixes like "-er" and "-est" without affecting the pronunciation or flow of the word. Using "more" and "most" allows for a smoother and more natural-sounding comparative form.

Third answer:

Using "more" and "most" for more-than-one-syllable adjectives maintains consistency with two-syllable adjectives that also use "more" and "most" for comparison. This avoids creating separate rules or patterns for different types of adjectives

Is there an identifiable reason for tending to restrict the forms -er, -est to single-syllable adjectives? It may be one of these I suppose; or something else.

  • 1
    Those three answers seem completely made up, ChatGPT-generated, or both.
    – alphabet
    Commented Nov 11, 2023 at 15:21
  • Related: More bored vs boreder
    – Andrew Leach
    Commented Nov 11, 2023 at 15:48
  • It is one and two syllable adjectives. Anything with three gets more/most or less/least. Funny, funnier, funniest. Beautiful, more beautiful, the most beautiful. This question belongs on ELL.
    – Lambie
    Commented Nov 11, 2023 at 18:22
  • Don’t limit yourself yourself to thinking about adjectives, because adverbs work the same way. Here’s an example of comparative adverbs: “Adjectives and adverbs form their respective comparative and superlative degrees in exactly the same way, so the sooner you notice this happening, the better you’ll understand it.” :) Please don’t take that sentence harshly; it’s just a demo.
    – tchrist
    Commented Nov 11, 2023 at 19:16
  • 1
    @tchrist Wow, the way you speak to people. Really too much.
    – Lambie
    Commented Nov 11, 2023 at 19:30