You're just not looking at Intel processors that are old enough :)

80286 needed no heat sinks, even 20-MHz models.

80386 used to do well without radiators, too, or with entirely passive cooling, even the faster DX variants.

Frankly, I don't remember low-power i486 (e.g. 40-MHz versions) to have heat sinks, too. You definitely wanted a heat sink, though usually a passive one, on a DX2 (66 MHz) or DX4 (100 MHz).