If we move for a minute beyond the direct "hard" technical constraints which have already been expressed here (i.e. small chassis, lots of chips etc.).
You have a second aspect, the old PBCAK (Problem Between Chair And Keyboard) one.
Users will use their laptops in less than optimal (from a thermal perspective) ways. They will use it on their laps. They will use it in bed, resting on duvets. They will use it on pool-side sun loungers with the mid-summer sun blasting down. Yes you can tell them in the manual that they shouldn't, but they still will.
So even if you can magically cool the chips passively, the possible use-cases of the product might ultimately constrain your ability to forgo a fan entirely.
I note that some people here are seeking to blame the "race to thin" for the requirement for fans. To them I would merely point them at the Panasonic Toughbook. Modern laptop but built like an old-shool 90's brick ... it still has a fan.[1]
Apple have done a stunning job with the M1 (and previously with the i5/i7). But even with the unibody chassis of a MacBook (i.e. the whole device is a massive heatsink) the necessity of forced cooling is still there (edit to add: with the exception of the MacBook Air M1 ... thanks for the correction guys !) . But with the non-Air M1s you really have to work super hard to make the fans ramp up at all, they've done a stunning job.