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