RAM memory is a volatile types of memory (such as DRAM memory modules), where stored information is lost if power is removed. RAM retains its contents while powered on but when the power is interrupted the stored data is immediately lost. That means even when you aren't using your phone the RAM is burning your battery. The more RAM you add the more battery you use. Your terabyte hard drive on you laptop is magnetic memory that doesn't require power to maintain memory that is used to load the RAM memory on yur Laptop since the Disk is too slow. A SD Card is Flash memory that doesn't require power to store information but it is too slow of memory to replace RAM. I replaced my Laptop's magnetic terabyte disk with a much smaller and more expensive Solid State Drive which is FLASH memory that is much faster than the rotating magnetic terabyte hard drive. Now I can boot my laptop is second and not minutes and every thing from the small Solid State Drive loads quick in the the fast RAM memory.
RAM cost a lot since it is very fast. As far as rotating computer disk, you have no idea how large in reality that a GB is HUGE. About 14 years ago I was selling a GB of IBM's SSA disks under Unix/AIX for about $1 million per GB. Disks back then were limited to 1.1 and 2.2 Mega Bytes not Giga Bytes. Also, back in 1980 I worked for a Computer MFG that sold 64 kilobytes of CACHE memory ( faster than RAM) for about $75,000.
RAM memory prices are already dirt cheap compared to 10 years ago but they will continue to get larger, faster and cheaper plus use less power. The only reason we are still using Magnetic and FLASH memory today is because RAM is small, expensive and requires power to store information.