Spider jockeys can spawn naturally (except wither skeleton variants) or by spawning monster eggs
But the command ride cannot make skeletons (including Stray, Wither Skeleton) ride spiders (including Cave Spider)

Affects multiple versions (possibly all versions have existed since the directive was added)

Observed Results:
Skeletons cannot ride spiders
Expected Results:
Skeletons are supposed to ride spiders

There may be other mobs that will naturally spawn versions like this that ride other mobs but the command /ride won't allow him to ride

Since I don't speak English, the above is translated by google This is the original text, I hope someone who can translate can help to add:
由於我不會英文 所以上面是google翻譯的 這是原文 希望有會翻譯的人幫忙補充:
蜘蛛騎士可以自然生成(凋零骷髏變種除外)也可以通過生怪蛋生成
但是指令ride無法讓骷髏(包含流髑、凋零骷髏)騎上蜘蛛(包含洞穴蜘蛛)

影響多個版本(有可能從該指令加入以來所有版本都存在)

Observed Results:
骷髏無法騎上蜘蛛
Expected Results:
骷髏因該要騎上蜘蛛

可能也有其他生物會像這樣會自然生成騎乘別的生物的版本但是指令/ride無法讓他騎乘