Why is bitcoin limited to 21 million?

The cap isn't written as a number. It falls out of the subsidy schedule: 50 new bitcoin per block in 2009, halving every 210,000 blocks. The series adds up to just under 21 million, with the last fraction arriving somewhere near the year 2140 — uneven block times mean there is no exact date.

Because amounts are whole satoshis, the subsidy rounds down to zero rather than ever reaching a clean 21 million. Changing the cap would need the same near-total agreement among users, nodes and miners that any rule change needs.