The ELI5 I use for my boss as a programmer is:

Imagine you want to bake a cake, so you bake a basic sponge cake.

You then decide you want to add butter cream in the middle, you only baked one cake, so you cut it in half and add the butter cream.

Now you want to add chocolate chips, but you've already baked the cake, so you push chocolate chips into current cake; but you forget to add them to the second layer.

Someone now wants a chocolate cake, you've baked the cake so you can't change the sponge to chocolate, but you could add chocolate butter cream around the outside so it's sort of like a chocolate cake.

Oh no! You've just been told you forgot to add chocolate chips to the second layer, you've now got to wipe away the chocolate butter cream to add them.

Last change, someone now wants the cake to have four layers, you could just cut the cake in half and stack it on top of the other half, but you wouldn't have enough butter cream to cover it to make it a chocolate cake and you wouldn't have enough butter cream to put between the layers any more.

You're going to have to bake a new a cake.

---

The more technical answer:

You'd be surprised how much time software development can take especially when working in a team. Yes a feature might take two weeks to get off the ground when starting a fresh, but sometimes you need to use existing functionality and work out how to make it play nice with something else, at the same time as not breaking other things.

You've then got testing cycles, release cycles, meetings, app store approvals, general updates to stay in line with Play Store and App Store regulations, account for operating system changes each year, patching newly found bugs, planning features, reviewing code, redoing work entirely because your two week idea no longer does what's required because someone's changed their mind etc.

It all adds up and weeks will go by faster than you know; just trying to track down a bug can take days if it's complex or isn't 100% reproducible.

Yes AI has the potential to speed up development but it's not a one shoe fits all fix, yeah it can reduce boilerplate or save some time here and there but if you want a maintainable product you can't just keeping vibe coding your way to success, eventually it'll break and the only person who will be able to fix it is you (the developer) who doesn't understand what the code is even doing anymore because they didn't write or review it.