It is the massive amount of space one car takes[1].That is a legitimate concern, but the kind of photo you linked to that is often used in advocacy is horribly misleading.
In reality, if cyclists rode anything like that close together, there would be accidents all the time because no-one would be able to make turns or travel at different speeds. The general advice to drivers here in the UK is that you should allow as much space around a bike as you do when passing a car, and authorities have conducted public education campaigns on this and even prosecuted drivers for passing too close. From a safety point of view, that advice is clearly sensible, as passing too close to a cycle is quite dangerous, particularly at higher speeds. But you can't have it both ways by claiming cycling is safer and healthier with that sort of space allowance but also that cycling is as space-efficient as it appears stacking bikes up close together like that. In reality, a bike is much smaller than a car, but in terms of proportions, a bike plus adequate space around it is only a bit smaller than a car with adequate space around it.
Buses are similarly very misleading in that sort of photo. That is partly because unlike typical car journeys they follow circuitous routes that may be 2-3x as far and as slow to get a single passenger from (somewhere near) A to (somewhere near) B. It's also because unless you're looking at an area with very high population density or very high usage routes/times, most buses don't travel anything like fully loaded but still take up the extra space whether they have 50 passengers, 20 passengers or none at all. If you're going to show a full-size car for each individual travelling, as these photos invariably do, why do they always show a single bus at exactly its maximum capacity? If you showed both at average occupancy levels, the photo would look very different.
This is without getting into the traffic modelling where you find uniform traffic speeds are much more efficient, but while cars can achieve something close to that on clear roads, it is heavily disrupted by both relatively slow-moving cycles that you might not be able to overtake and stop-start buses that might block a traffic lane while allowing passengers on and off.
There are very practical questions to be asked about why we still drive around in vehicles with space for 4-5 people and luggage capacity for going on a two week holiday even if we're travelling alone and lightly loaded, but a lot of the anti-car arguments made by pro-cycle and particularly pro-bus advocates using the kind of photo linked above are rather disingenuous.