Scifi ships tend have engines with stupendous power ratings (Gigawatts and higher). Even with 99% of efficiency they'd have to shed megawatts of heat. Since there is no convective or conductive cooling in space your steady-state option is radiative cooling which is constrained by surface area, temperature difference and emissivity. You don't have much flexibility on the latter two. So you need to increase surface area. The ISS' radiators take up about 200m² and can reject 70kW. There are non-steady-state options such as sacrificial coolants or plain thermal mass but that would require extra mass, resupplies or additional downtime. In The Expanse the ships just don't have enough area considering they're powered by fusion reactors burning constantly.