Why do firms maximise profit when marginal cost is rising?

Because to maximize profit you keep on selling until marginal cost equals marginal income(MC=MR)

Some Key points

• MC is the increase in total cost from producing one additional unit.
• The MR is the increase in revenue from the sale of one additional unit.
• One way to determine how to generate the largest profit is to use the marginal revenue-marginal cost perspective. This strategy is based on the fact that the total profit reaches its maximum point where marginal revenue equals marginal profit.