And why is this a good thing?
The only 'excuse' I can think of in London is that the Tube is very full, so they want to encourage some people to use the bus to relieve the Tube a little.
That and demographics, i.e. the bus is far cheaper which means those who can't afford the Tube have an option.
However that isn't a reason why an interchange shouldn't be free, nor why there shouldn't be two fare levels, bus only and everything, rather than a fare per mode.
The answer is no doubt "because we've always done it that way", with a measure of "because the accounting is easier" and "because some people would whine if we changed it". Oyster was a wonderful chance to redesign the fare structure, and contactless another, but instead they overcomplicated it by making it deal with the nuances of a fare structure built up over time rather than designed.
Neil