Human beings have physical and cognitive limitations - these differ among individuals with a degree of variability, but there are hard limits.
Hardwired cognitive limitations exist when it comes to attention, perception, memory, the complexity of mental calculations they can perform, and how long we can do that for without rest.
Humans have physical limitations that need to be kept in mind - if we do not eat, sleep, rest, and maintain health for physiological wellbeing, our capacity for physically doing the work is impacted, as is cognition and problem solving capacity - physical exhaustion is a large factor in degrading cognitive performance, because working memory and cognition are actual meaningful processes that require a lot of the human body's energy.
Humans can handle complexity in circumstances very well and navigate it fluently - in fact our in the moment problem-solving and adaptation is one of our key strengths, and is likely the reason that we are the last surviving species of humans.
Related to the above two points, human beings are also exceptional at learning, and actually can genuinely feel rewarded when learning. This said, learning requires even more energy and resources, and subsequent rest. Doing so ongoing, without the opportunity to engage in familiar activities that we have a sense of mastery over, can cause long term dips in performance.
Human beings also have limitations in their senses, which impact on their ability to make sense of the world and adapt to it. These extend beyond the traditionally well known senses of sight, hearing (auditory), smell (olfaction), touch, and taste (these are the exteroceptors), to our experience of internal sensations like pain, temperature, internal processes and organ sensation (these are interoception senses) and our sense of position and movement in physical space (proprioception).
Humans evolved as highly emotional creatures, and emotion still dominates our thinking, experience, world, and behaviour, profoundly - our emotional experience is what makes us human, and we cannot turn it off or 'leave it at the door' - emotional experiences and signals are integral to cognition and rational decision making.
Emotions therefore can act as a highly impactful performance accelerator, and also handbrake - it is critical to expect emotions, and to work with them accordingly - as with physical exhaustion, emotional exhaustion is also possible, common, and significantly dibilitating.
We need simplicity in our work design, processes, and systems - diffusion of goals and targets dilutes energy and capability - human beings thrive on clarity and surety.
Most people, most of the time, are trying their best, and trying to do the right thing - for the most part every human is acting and reacting in a way that makes sense to them - nobody is walking around doing stuff that they think doesn’t make sense.
As a species we evolved as a social and collaborative group, and depend in real terms on feeling a part of the group/community, to be seen and accepted - if we do not feel like we belong, we will respond with emotions that are not helpful - belongingness will impact feelings of psychological safety (by definition), meaning & drive, and how we relate to and interact with others.
Modern neuroscience research actually outlines a 'metabolic tax' of exclusion, showing that when somebody feels like they do not 'belong, their brain enters a high-alert state, siphoning energy away from problem-solving and focus, creating further drag on our physical capabilities and limits.
Feelings of physical and psychological safety are hard requirements for human beings to perform to their potential, sustainably, and ongoing, and is highly related to their sense of belongingness.
It is worth mentioning that as a species, human beings are not really built for the world that we have built for ourselves, considering the points raised above.

Sources (selected)

Miller (1956)

Baumeister & Leary (1995)

Baddeley (2012)

Cacioppo et al. (2021)

Psychological safety