I don't know if anyone has thought about this question:
Paladin was originally a subclass of fighter, which is regarded as a warriors, but the actual setting is more like a mixture of priest and fighter; the basic settings of monk (ab, Hit point dice, etc.) are regarded as priest, but the actual setting is more than paladin is more like a warrior: the abilities are all specialized for melee combat and without any divine spells.
That is to say, paladins are considered warriors, but they are set like spellcasters; monks are considered priests, but they are set as pure melee guys. This should not be right.
Paladins should be melee warriors, and monks should be more melee priests at most, so the logic makes more sense.

I think one needs to consider the cultural upbringing of the creators of D&D.

Paladins are based on the medieval warrior knight classes such as the Templars and probably most especially the Knight Hospitallers. Hence the ability for healing magic like clerics.

Monks on the other hand were probably influenced by movies and the 70s TV show Kung Fu which has an Eastern monk using his martial arts skills while searching for his family in the 19th century American West.

Maybe it can still be slightly improved: paladins can be changed to have more immunity/defensive feats like monks and spells can be reduced. In fact, the spell slots of paladins are not enough; monks can be changed to have divine spells, and remove some special abilities.

BTW, Paladins originated from Charlemagne's 12 Guards, although it is certainly possible that he was influenced by military orders such as the Knights Templar.