It's physically easier to touch the reader than to hover over it? Especially if the reader is a bit janky/older and takes time to complete the transaction. If you look at Oyster (TFL) for example, most people physically touch their cards to the turnstile/bus readers. The official advertising also uses the phrase "tap in, tap out" for stations where there are no barriers. "Contactless" is also fairly widely used in the UK - e.g. "do you take contactless?". Contactless is because in theory you don't need to touch your card to the reader. My new phone seems to have a good enough antenna (active?) that it triggers the POS terminal much further away than my debit card, but usually the activation distance is small enough that tapping is easier.