Are devs somehow cheating at these games made for retro consoles? They look so much nicer than games made during the console's era.
Compare to Amstrad CPC Sonic, for a similar 8-bit machine—but only the final and most expensive models.
But a big thing is that back then, games were often made on a timescale of months, and with a budget. Hobbyist dev has no limits to how much time they want to sink into doing something properly. Ideas & techniques have had many years/decades to percolate into existence.
1) Transfer a block of data from main memory to expansion memory
2) Transfer a block of data from expansion memory to main memory
3) Exchange a block of main memory with a block of expansion
4) Verify a block of main memory with a block of expansion memory
copy is ~1MB/s compared to manual CPU speed of <120KB/s