By design.
In OD&D and AD&D, adventures were crafted around 6-8 characters, it was not uncommon to equip underlings to boost those party numbers, and characters who died restarted at level 1, but could catch up in the unique XP system fairly quickly if they could stay alive...which would be due to having magical items handed down to them. Because of that, a well-geared 1st level fighter could hang with a 6th level party. Magic items and weapons were commonly found at level 1. Bounded accuracy and balanced class design were not part of the conversation. Finally, magic items could be broken and destroyed, and if a DM wanted to ensure no players returned to his or her table, see Mordenkainen's Disjunction.
3rd Edition: the "Christmas tree" effect where numerical magic items to boost stats became a necessity. Wealth by level replaced hoards of gold with hoards of "plus" items that would be sold towards the next tier of stat-boosting items. Easily my least-favorite time of magical items.
4th: skipped
5E: because of bounded accuracy and power imbued within the classes, magical items are less necessary. Because of the math, stat boosting items have more impact than before. There was/is a push to make each and every magical item you put into the game special so players don't yawn, toss it onto the heap of +1 swords, and so on.
I do recall a thread years ago about adventure design and the expectation not all encounters would be had, so if one were to scour the entirety of a prefab adventure, there would tend to be more magic than written into the DMG/Xanathar's expectations.
Summary: the TSR era handed out magical items like candy at 1st level and never stopped. Without attunement, the only thing stopping you was the 2-ring limit.