For reference, the description of Attunement (from DMG p. 136) states:
Attuning to an item requires a creature to spend a short rest focused on only that item while being in physical contact with it (this can't be the same short rest used to learn the item's properties). This focus can take the form of weapon practice (for a weapon), meditation (for a wondrous item), or some other appropriate activity. If the short rest is interrupted, the attunement attempt fails. Otherwise, at the end of the short rest, the creature gains an intuitive understanding of how to activate any magical properties of the item, including any necessary command words.
An item can be attuned to only one creature at a time, and a creature can be attuned to no more than three magic items at a time. Any attempt to attune to a fourth item fails; the creature must end its attunement to an item first. Additionally, a creature can't attune to more than one copy of an item. For example, a creature can't attune to more than one ring of protection at a time.
As the DM, you are more than welcome to remove the attunement requirement or edit items in other ways. I don't recall any specific rituals for doing so in the rules.
In my opinion however, there is a cap in the number of attunements you may have so that players cannot use a potentially-infinite number of magical artifacts simultaneously. If you allow players to use powerful items without attunement you may have problems with balance.
Also, it makes sense for a dragon-slaying sword to not always be able to slay dragons because the magic inside it deals the extra damage, not the physical sword. Someone who is not attuned would not be familiar with its magic and would not have "an intuitive understanding of how to activate any magical properties" and would therefore only be able to wield it as a regular sword.