You can print unlimited banknotes, but you cannot print unlimited purchasing power. If every household suddenly received a million pounds in cash, the price of everything would simply rise to match the new supply of money. Nobody would be richer in real terms — they would just be paying more zeros for the same goods.
This relationship between money supply and prices is captured by the quantity theory of money: when the amount of money in circulation grows faster than the real economy's output of goods and services, prices rise. This is inflation.
In mild doses, inflation is manageable. Most central banks target around 2% annual inflation — enough to keep the economy moving without eroding savings significantly. But when governments print money to fund spending they cannot afford through taxation, inflation can accelerate into something far more destructive.
The extreme version is hyperinflation. The most famous modern example is Weimar Germany in the early 1920s, where the government printed money to pay war reparations. Prices doubled every few days. People literally wheeled barrows of cash to buy bread. The German mark became worthless — not because it was not legal tender, but because no one trusted it to hold value from hour to hour. Zimbabwe in the 2000s and Venezuela in the 2010s experienced similar collapses.
Printing money also redistributes wealth in a hidden way. It effectively taxes everyone who holds savings in the currency — their purchasing power quietly erodes. Meanwhile, those who hold real assets (property, stocks, commodities) see their values rise in nominal terms. It is a transfer from savers to debtors and asset owners.
The fundamental reason governments cannot print their way to prosperity is simple: money is not wealth — it is a claim on wealth. You need the real goods and services first. Creating more claims without creating more goods just means each claim buys less.