They simply didn't let you send much of a bitmap
The first HP LaserJet printers on the market required you send jobs via PCL language. The language was tuned to pretty much not let you do anything the printer couldn't handle. PCL for the whole page needed to fit in the very limited RAM.
Needless to say, PostScript was out of the question on these early printers, as PostScript was architected for absolute peak performance"professional typesetter" level functionality at all costs, full knowing this would make a page buffer necessary and dramatically drive up the costs of the printer's rasterizer. Adobe was not after the low-end market.
Yes, the onboard CPU on the printer was rasterizing PCL "on the fly" trying to keep ahead of the drum/laser. First, this was nothing new: video games were already doing that. Second, you were married to the fonts HP provided in the printer, and nobody minded that because every printer up to this point did exactly the same thing.
While the language tried to discourage it, and application developers tried to avoid it, one "could" create a job that the printer could not print for lack of ability of the CPU to keep up with the laser raster. This would end in failure.
Apple's first printer was a totally different deal. That did come with 1MB of RAM and that was used to pre-rasterize the entire page. It was a really big deal, and added to the cost of the printer. It then came with PostScript, which could do all the modern stuff you expect from printers today. Note that the PostScript could take literal minutes to rasterize a page, even though the LaserWriter CPU was often more powerful than the Mac CPU.
It wasn't until the HP LaserJet III that it had (or could be made to have) enough RAM to rasterize a whole page. And then yeah... you could get a PostScript cartridge that went in one of the font cartridge slots.
Font cartridges. Really. It was like that.
This didn't offend the market because Windows hadn't "arrived" yet. PC screens were 25x80, character cells were 8x8 (except the lavish IBM MGA which was 9x14) and PC display fonts still came from the video card hardware. Printers up til now had the font(s) they had. So people were not picky. The text "character" was still the basic currency of computing.