On commercial channels, prizes are funded (in part, at least) from advertising revenue. With far more TV channels than there used to be having to share the available revenue between them, and with many advertisers moving away from expensive TV advertising to cheaper (and often more effective) social media promotions, ITV has seen over a 90% drop in its 'real terms' income from advertising and struggles to survive. So they won't be rushing to give large sums of money away in prizes. (In practice most quiz shows are made by independent companies who have to sell them to commercial channels. They need to keep their prices low in order for those cash-strapped channels to afford them, so they have to keep their own expenses as low as possible).
The BBC has always eschewed the use of high-value prizes for its competitions, preferring to see the pleasure of taking part, and then hopefully of winning, as reward in itself. (Double Your Money based its format on the US show 'The 64 Thousand Dollar Question". The BBC regarded such things as anathema to them and their viewers, regarding them as distinctly 'un-British'). For example the winner of Mastermind receives nothing but the trophy.