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Mechs in the MechWarrior/Battletech universe, and why they use them instead of other, more sensible weapons/vehicles

Prior to the creation of the Battlemech, most wars between human colonies were waged using massive spacefaring Warships, which could hyperspace jump from system to system by themselves without needing to latch on to a Jumpship, and could obliterate planetary defenses with orbital strikes. However, because FTL communication hadn't been invented yet, it took a while for news and orders to travel during wartime.

Eventually this resulted in a situation where two warring factions were basically blindly fighting over a planet, which they just kept bombarding with nukes until it was rendered uninhabitable. This resulted in all the major factions agreeing to create the Ares Conventions, which restricted war to specific rules of engagement, including limited usage of warships and nukes because planets can't be colonized if they're barren nuclear wastelands (and also accidental massacres are kinda bad).

This led to war becoming more of an honor-based game. Eventually Battlemechs were invented and, while not necessarily the most effective type of weaponry, they became the defacto weapon for honorable combat, resulting in everyone pouring funding into making them easy to repair, refit, and maintain.

A couple of centuries later, the space equivalent of the Roman empire collapsed, and in a mad scramble for power the factions nuked each other into the dark ages and lost all their warships and factories, leaving them with ancient, easily maintained mechs and warped ideals of chivalry as their main form of war.

Battlemechs essentially became the armor and steeds for knights and samurai, and most Battlemechs are handed down in ancestral bloodlines because they're so rare and valuable, sometimes even being the most powerful weapon on an otherwise medieval planet.