Why would humans trade smell for sight and sound?
The clearest answer in the supplied evidence is an evolutionary one. Olfaction and motivation were closely linked earlier in evolutionary history, and the human shift away from relying on olfactory stimuli toward visual and auditory stimuli is presented as part of that story [2]. In plain terms, smell may have mattered more to our distant ancestors than it does to us now, and the senses we lean on today are the ones that took over that role.
This is a proposed evolutionary framing rather than a demonstrated cause. The evidence describes the shift but does not spell out the pressures that drove it, so the honest reading is that a shift occurred and that it is consistent with reduced reliance on smell, not that we know exactly why it happened [2].
Smell is diminished in priority, not switched off
A smaller role does not mean no role. One paper notes plainly that we cannot rely on smell alone, which frames smell as one channel among several rather than a primary guide [3]. That matches the broader picture: vision and hearing carry most of the load, while smell contributes alongside them.
Smell also remains tied to emotion and motivation in humans, the same link that the evolutionary account describes as older than our visual and auditory habits [2]. So the useful way to think about it is a change in priority, not a loss of function.
What the evidence does not settle
Several of the supplied papers touch on human smell without explaining our limited reliance on it. Work on obesity and diabetes examines how metabolic conditions change odor thresholds and olfactory perception, but it addresses how well people smell, not why smell plays a smaller part in our lives [1]. A paper on scent in social contexts looks at how smell shapes interpersonal perception, again without offering a reason for reduced dependence [4]. A discussion of odor sensation and memory covers detection, sensitivity, and false odor perception, but does not explain why humans lean less on smell [5].
So the direct answer rests mainly on the evolutionary shift described in one source [2], supported by the observation that smell cannot carry us alone [3]. The other papers describe what human smell does and how it can change, which is useful context but not an explanation of the "why."
Sources used in this answer
Olfaction in the context of obesity and diabetes: Insights from animal models to humans
Obesity and type 2 diabetes are associated with changes in odor thresholds and olfactory perception in humans and animal models, but the review does not address why humans rely less on smell.
Olfaction and emotion
Olfaction was closely linked to motivation earlier in evolutionary history, and humans shifted toward relying on visual and auditory stimuli instead.
Perspective-Taking toward Nonhuman Others: An Odor-Focused Environmental Education Program Promoting Anti-Anthropocentric Views
An odor-focused education program notes in passing that humans cannot rely on smell alone, without explaining why.
Can I smell YOU?
Human scent perception matters in social and interpersonal contexts, but the paper does not explain reduced human reliance on smell.
Odor sensation and memory
Odor detection, sensitivity, memory, and false odor perception are discussed, but the paper does not explain why humans depend less on smell.