They did have time, they simply couldn't be bothered.
At the start, the Atari ST version of a game was outselling the Amiga version, assuming that the Amiga was even getting a version that is, it was never a guarantee the Amiga got a conversion.
Most early ports were just quick and dirty ST ports, the graphics were already designed for the lesser pallette on the ST, about the only thing that did get updated was the music.
And there were not that many development teams that were truly multi platform capable. So, a software company would award the conversion contract to whomever could convert to as many formats as possible.
So you have to consider in the early days of the Amiga, the following also got a conversion:
Sinclair Spectrum, Amstrad CPC464, Commodore 64, occasionally PC, ST, Sometimes MSX, Sometimes Nintendo NES, Sometimes Sega Master System.
So if ONE development team were responsible for coverting that lot to all those different formats, they didn't have the time to properly utilise individual machines.