It seems seems to be a capacity issue.
If you look at the models used in qigong, energy, you could say, has types. The three main types of qi are cosmic, earth and human.
Humans get energy primarily from food and breathing air (earth qi). We can also breathe in energy directly from the cosmos and stellar bodies and planets if we choose to and process that. We turn to turn it into human qi.
Astral parasites are unable to process cosmic qi or earth qi for food.
Human real vampires are also unable to process the energy of the cosmos or earth well, and become sick from qi deficiency. This is why they are vampires. Qigong and Michelle Belanger, an actual real vampire after in this. All other self-described vampires are technically energy workers. They are not feeding to stay alive, they are cultivating qi from humans instead of the cosmos. (As an energy worker, I see this as an inferior and dirty energy source, but it has its uses).
This is why astral beings that can only feed on human qi attack humans to get it. We actually radiate qi a lot and most astral beings feed ambienly.
However, as above so below, when we die we leave the wi body discarded, same as the physical body. As fungi and bacteria recycle the physical, there are also astral beings that recycle qi bodies, such as the “larvae”. Larvae are large leech-like things the size of dogs that are very dumb and are attracted to dead qi to recycle. Sometimes, if a live human is sick, in excess or leaking qi, a larva can attach to them by accident. You get the same drained feeling, but it’s not malicious and you can scan yourself and pluck them off. This is actually very common.
They fall away naturally if you regulate your qi and stay balanced, which is your qi equivalent of having a good immune system.
And then some things just want more or need or like the “flavor”, as it were, of specialized energy in the form of emotions pr sexual energy (succubi or incubi).