Why Replies Outweigh Likes on X
A like on X is close to free. It takes a fraction of a second, leaves no trace anyone reads, and does not require the user to have an opinion. A reply is public, attributable and permanent. The person writing it is attaching their name and their account to a position, in front of their own followers.
Ranking systems weight signals roughly in proportion to what they cost to produce, because expensive signals are the ones that are hard to manufacture. When X open-sourced parts of its recommendation code, the published weightings reflected exactly that ordering, with replies sitting far above likes.
The behavioural evidence is easier to check than the code. Watch what happens to a post with an active reply thread: it keeps reappearing, gets pulled into notifications, and stays alive for hours. A post with likes alone does none of that. Every reply is a fresh event the system can act on.