Best ELI5 way I have heard this described.

Imagine a safe company who makes the best safes in the world. So good that the owner can open it super simply, but it would take anyone else their entire life to have a .00000001% chance of getting inside. The lock, the construction, the materials are all so good and built so well and it's so cheap that eventually every person in the world owns one.

Every person has one and they keep all of their most private items in it, and since it's impossible to get into they are very careless with where they leave it, who sees it, who knows where it is, or who they leave alone with it.

Then one day a person does something really bad, and the government gets a warrant to search their home, and like every other person on earth they owned one of these safes. The government justly and legally takes possession of the safe but they cannot get into it. No matter how hard they try the just can't get inside.

So they call the safe company and ask them a bunch of questions about the materials, the lock, the construction, and ask them if they know any easier way to get in. The company tells the truth and says "we don't know, we build them so no one can get into them, not even us"

Then the government asks them to put a door on the bottom with a regular old key, and give a copy of that key to them for super safe keeping, perhaps they even put the key into one of the safes. They say that they will keep the key safe and only use it if it's a national emergency.

But the problem is that if you put a regular door with a regular lock on the bottom, anyone could pick that lock and gain access to the contents of the safe, with our without the key. If the company did what the government asked them to do, then every single valuable item possessed by every person on earth could be stolen by anyone who bought a $10 lock-pick set and took a few days to learn how to pick a lock.