Three reasons, and they stack.
The first is the 80 per cent line itself. Borrowing above it is possible, and it brings lenders mortgage insurance with it, which is a real cost rather than a technicality. We cover when paying it is a reasonable call in our guide to
lenders mortgage insurance.
The second is the valuation. A lender values the property itself, and that figure can land below a portal estimate or a neighbour's sale price. The valuation is the number the calculation runs on, not your estimate of it.
The third is the one that catches people furthest along: equity and borrowing power are two separate tests. Releasing equity tells you where a deposit can come from. Whether your income supports the larger total debt is assessed separately, against the lender's own buffers. Clearing one and failing the other is ordinary.