Because I am not certain about anything. That includes uncertainty about the usual strongly held beliefs (until new evidence appears) about the nature of reality that atheists tend to hold. The fact is that we cannot accurately perceive our existential situation, other than in the abstract, and still be functional. The stimuli would be overwhelming. Thus our perception is severely filtered. What happens when a filter breaks? It becomes distorted, thus mental illness and/or sensory dysfunction. What if the filter is completely broken? Everything it was filtering out is let in. That may be what's happening in some NDEs.
The anecdotes are on record and easy enough to find. Anecdotes are the weakest form of evidence, and this has been a huge problem in psychology and psychiatry which, as with this question, focus on the subjective. How to be objective with the subjective? In psychology, the attempt to resolve the problem resulted in Skinner's behaviourism, which added rigour to a field that had until then been thought of as woo-like. Yet it is limited, not taking agency into account.
When a patient is dying, as with sleep, their world shrinks - bed bound and, as the senses weaken, the subjective is all they have. The objective has nothing of interest - pain, nausea, flashes, beeps, bangs and footsteps, vague voices - and that's when the subjective intensifies, as evidenced in sensory deprivation tanks.
An observer just sees poor old Joan dying in a hospital bed, with numbers applied to various measures like heartbeat, blood pressure and temperature. At that time Joan seems more or less the same as any other old person in bed, covered in tubes, but who can say what's happening in her inner world? She could be comatose, conscious and locked-in, dreaming, or could be having a mind blowing experience of significant import should she survive.
The next step would be to take brain scans but, it's hard to test for. Firstly, transitory phenomena is notoriously difficult to study and, secondly, intrusive measures unrelated to healthcare are not ideal to impose on the dying without their prior consent.