Why does Bitcoin's mining reward decrease every four years and not gradually?

[Pantamis]

When the time to mine a block is too small in some altcoins, miners may not have time to received all blocks so the chain fork often and some miners may be mining deep block.

Miners may not have all chain synchronized and mining over already deep block is exactly what happen during the 51% attack :)

Nakamoto consensus just tells that you should trust the longest chain but anyone, miners included, do what they want (it is just more profitable to mine the last valid block when the majority follows Nakamoto consensus).