Define “high precision”. You cannot buy ±0.00% references, and once you get below 0.1% (*) things can get expensive quickly (e.g. AD586 at 0.05% is $10 ea or more, and for even more precision you may need more complicated circuitry that’s harder to build and source).
So reasonably cheap and widely available reference + a bit of trimming seems like a reasonable engineering tradeoff, and keeps the budget within bounds.
*) I’ll leave it to @antoine.pasde2 to calculate the lowest tolerance you’d need to skip the trimmer