Picture a tall stone wall that has begun to lean. Someone builds a thick block of stone against the outside of it, and the wall stops moving. That block is a buttress: not part of the wall, but a separate solid thing, placed where the push is, pressing back. Every use of the word keeps that shape — one thing standing beside another and taking its weight. When what is leaning is an argument instead of a wall, the evidence you add is doing the same job, in the same place.
From the word’s history
The word goes back to Old French ars bouterez, literally supporting arcs, from a root meaning to push; it is ultimately cognate with beat. So the pushing back is in the word from the start, not just the standing beside.
這個字可以追到古法文 ars bouterez，字面是「支撐用的拱」；bouterez 再往前接到一個意思是「推」的日耳曼字根，跟英文的 beat 同源。也就是說，這個字從一開始講的就是「抵住、往回推」的那股力量，不只是「放在旁邊」而已。
Wiktionary kaikki extract, source_ref kaikki:3f6f69a190036ae4:1249012406:27191 (Old French ars bouterez < bouteret < Frankish *bôtan < Proto-Germanic *bautaną, to push)
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