Well then , Why do SSDs have to rewrite a whole block (512KB) just to change a page (4KB) or two ? is it just for the sake of drive lifetime consideration? , or Do they have lower access to each page than HDDs for example?
That's just how they're built, circuit wise. The data retention mechanism is the charge/voltage held in each cell. The individual page writes can only drain charge (i.e. change 1's to 0's).
Adding charge is only done by refreshing the whole erase block.
I assume the disparity between erase block and page is due to circuit complexity and density requirements. It just isn't feasible to have byte or even page addressable reset capability.
This generally isn't a huge problem, though, and doesn't hurt you at all in big linear files, and the cost can be hidden for smaller writes through caching. It begins to generally fall apart when you start ejecting blocks from the cache due to overflow and then rewriting pages within them (for example, cycling through writing one page every block on a very big file).
I haven't been in the industry for about 5 years, but any voltage control, etc. was inside the individual NAND chip (each NAND has its own controller). The key to speedup was managing the virtualization of pages/blocks, doing parallel writes (i.e. RAID among the NANDs), and having enough cache to hide the extremely long write erase times.