This is exactly the point!

SRAM is very easy to use - just connect it to the address & data busses - but expensive per byte;
DRAM needs all that refresh controller stuff - but is cheap per byte.

So if you need very large amounts of memory, all that refresh controller stuff is only a minor inconvenience compared to the very large saving in the overall, total memory cost;

However, if you need only small amount of memory (and that covers anything practical for an 8051), the hassle of DRAM is simply not worth it as it makes the overall, total memory cost higher than using SRAM.

So that's why PCs use DRAM for Gigabytes of memory, while 8051s use SRAM for Kilobytes of memory.

SRAM can also be made much faster than DRAM (although that makes it even more expensive) - and that's why PCs use a small amount of very fast SRAM to cache the data from their very large, relatively slow DRAM.