Why is there a 999 limit on the maximum amount of armor?

[ANZ]

I started with Ironclad and tried to go with defense oriented build. But it didn't went well and I was barely surviving on the first two floors.
Fortunately I got a Council of Ghosts event almost right before the second Boss and managed to beat it with 8 HP remaining. And finally got a Barricade card from him.

Then I got to the final boss with almost no problems and slowly managed to accumulate 999 armour in fight. I was sure that I already won but then it went into dealing whooping 864 damage in one attack!
https://i.imgur.com/P05qUrL.jpg
Thankfully I beated it right at this turn, but if this had prolonged on few more turns then I could have been wiped out.

It's especially annoying because I could have got few thousands of armour if not this restriction.
It would have been pretty lame and unfair to lose because of this :/

[DJDiceZ]

It's an arbitrary limit, and it's true it can be troublesome if you stall the heart fight too long, but aside from that and endless which isn't really relevant to the design of the game, you never really need more than 999 block, so it's not a big deal (and you still winning despite being in the fight for so long further reinforces that point). It'd be an extremely rare occurence if you could even beat the third boss and still take too long to kill the heart before it breaches the block cap, so it's almost never gonna happen and it's not a worry you need to have, at least for the wide majority of runs. But i understand that it's annoying to not be able to go as high as we want even if just for the satisfaction of a big number.

I think i've seen some mods that disable the limits but don't quote me on this.

Don't worry about the screenshot for proof, as a scaling enemy the Heart's damage can pretty much ramp up to infinite values so it's not surprising it dealt that much damage. Plus it gets there decently fast.

[DuckSonata]

If the block limit was something else, the devs would just have designed the heart to scale past that limit instead. They want the heart to counter turtle decks.

[1.2M | Missingno.]

It's a kill screen for taking too long. You're supposed to kill it well before it gets to that point.

[Zu]

Rember when the heart had no damage cap? Those were the days.

[Spawnling]

You're not supposed to take 17+ turns to kill the heart.

To answer your question: The 999 Block limit is there, because enemies are supposed to out scale you at some point and kill you. The game devs wanted it this way.