While there are reasons gates may bit be known for sure until late, it is also policy of the investment funds that run LHR and LGW not to show your gate until a set amount of time before the flight, 45 mins if I recall correctly (hence why every flight by complete coincidence will have its gate known the exact same amount of time before departure, unless there are problems).

It's not a conspiracy theory that this is done to encourage people to spend their money, unless you think there genuinely isn't a single flight they have ever known the gate for more than 45 minutes before departure. It will also happen for the first flights of the day when the planes are already at the gate.

It is annoying, especially given the whole airport experience makes a lot of people very anxious and this doesn't help. It's something that doesn't seem to be done anywhere else in the world where pretty much everywhere I've been will tell you where to go well before departure so there's no last minute dash. Even the spiritual home of capitalism the USA in the airports I've been to.

Factor in that if you're flying EasyJet they say gates close 30 minutes before departure, and you create a rather small window to get to the gate and a lot of unnecessary stress for people, many of whom are not as experienced with airports as those on this forum.