Usually when we talk about money, time, distance, or other quantities such as volume (litres, cubic metres, etc.) or weight (kilos, tonnes, etc.), we are thinking of the entire quantity as one "lump sum" rather than the individual parts of that sum.
Look at another sentence:
(4) Ten pounds is on the table.
I'm looking at the whole amount, something that is called ten pounds. It might be one 10-pound note, it might be two 5-pound notes, it might be one 5-pound note and ten 50-piastre coins, it might be forty 25-piastre coins. The parts are not important. The sum — ten pounds — is important and that quantity takes a singular verb.
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