Like with many engineering design decisions, there is a point of which it takes exponential cost/effort to achieve minor incremental gains. While panel gaps may be a indication of quality in other areas (eg. if an OEM was attentive to panel gaps, it's likely they've put similar effort to other areas), it doesn't really offer that much value past a certain point.
Other than visual, the only direct and meaningful impact is wind noise. But that can be mitigated with a rubber seal which is cheaper and works better than trying to achieve a 1mm gap. Interestingly, on my 2024 Outlander I noticed they used a rubber seal around the front door panel gap to minimize noise but not on the rear doors (to save cost). I personally would prefer a vehicle with a 3mm gap that's filled with a rubber than an open 1mm gap that looks good from afar.