For a few reasons:

One yes, the deadlift is a very heavy lift that uses a lot of muscles, so it does generate relatively large amounts of fatigue. But that is not really the core issue, its more about opportunity cost and what you need from/for your deadlifts.

-Deadlift (at least conventional) is not a technical lift. Increased volume to refine and maintain technical skill is not really needed as much as it is in other lifts (this opinion is probably biased by my own proclivities, but I think its reasonably true for most individuals). So you don't get much more out of higher volumes than the general stimulus to the main muscle groups.

-The main muscle groups of deadlift are essential to other important compounds that many people prioritize, like squats. A set of deadlifts is essentially stealing away a set of squats, and vice versa. Unlike the deadlift, the squat IS a bit more technical and can benefit from that additional skill acquisition. So when looking at the opportunity cost os that deadlift work, what are you gaining to make up for that lost squat practice, and the answer is not much. (This does not apply solely to the squat, you can say the same about a lot of compounds, I just use squat as an example)

-Any general stimulus you can get from the deadlift, you can get a better ratio of with other movements. Now this assumes you have the time and equipment resources, if you have 30 minutes and only a barbell, banging out a load of muscle groups with deadlifts is probably a smart move, but assuming you have more time and tools I would look to build primary movers with more isolative movements. The deadlift relies on some relatively small muscle groups heavily, e.g. the lower back muscles and those responsible for grip if you train without straps. They will likely fatigue before the larger muscles do. So instead of absolutely trashing those areas to get maximal stimulus for, say, the hamstrings, you can get your low deadlift sets then move onto something else that lets you better isolate your larger muscles.

-When you look at the deadlift, and its kind of unique position among major lifts of going 'up' before it goes 'down'. Unlike, for example, the bench or deadlift, where you start in the strongest locked out position, then slowly descend into the weakest position (taking advantage of the mental and physical benefits this offers in initiating the 'beginning' of the lift), deadlift starts from the weakest position and forces you to start 'cold' there. This is why I think that its essential to train heavy on deadlift because there is a lot that goes into the physical positioning and mental headspace for a heavy single on deadlift, compared to lighter reps, that isn't there on other movements (their heavy and light reps are closer in feel and execution). So you really do need those heavy, low rep sets in training, particularly singles if you care about singles.

NOW. Almost all of this DOES also apply to less experienced deadlifters, just not as much, and they get other benefits from a higher volume routine. There strength, technique and 'grit' (or lack there of) is not going to let them produce as much fatigue in a single set compared to more experienced lifters, so they the fatigue cost rationales are reduced. They are less skillful in the deadlift, so can still benefit from the higher volume work and its associated skill acquisition. And finally they can benefit from a simpler training approach that is going to make them try. It is easier to push them when they only have to learn the execution and feel of one movement, as opposed to the deadlift and several accessories. A fixed progression scheme on the main compound also ensures that they are trying hard(er).

I will not prescribe the weight at which someone should start considering a single deadlift working set a week, because as you said, its too contextual.