(Microeconomics) Why must fims always lower prices when there is excess supply?
You are absolutely right. In many markets, especially consumer markets, which people often think of, firms are not forced to lower prices. If a bakery produces to much bread one day, it might make a bigger profit if it keeps the price and throws away the breads they have not sold at the end of the day. That is because its limited competition.

What many forget to tell you when they teach microeconomics, is which markets microeconomic models is designed to model. It is market with perfectly competition (infinitely many suppliers and consumers) and where everyone have perfect knowledge about prices. Think if big international markets of standard products, like crude oil, flower, rice and stocks.

If you are a farmer wanting to sell rice on an international exchange (normally you will just sell directly to a distributer), you are to small to affect the market. Your only option is ether to sell a tiny amount below the spot price (the last transaction), or choose not to sell at all (something you probably don't want to do). If there are more sellers than buyers, the price is pressed downwards. This downward price pressure will continue until prices are either so small that buyers want to buy more, or the price is so low that some producers don't want to sell anymore.

If you did not understand the last paragraph you should probably read up on how stock/commodity market auctions work.