When CDs and access to the internet became popular in the 1990's, people had ample access to e-books, but they mostly remained curiosities. The CD bundles that were so popular often came with CDs packed with e-books. People liked the idea of e-books, but not many people wanted to read them on their computer.
There has been more than enough room to store e-books on computers for a very long time, vast servers were not required. The big problem with cloud storage is that if you don't have access to the internet, you don't have access to your books. Local storage space is cheap. With a couple gigabytes, you have access to a vast library of books. It is getting hard to find a flash drive with a capacity of less than 2 gigabytes, at Best Buy, they don't show anything less than 4 GB.People tend to have their MP3s stored locally, and e-books are considerably smaller than MP3s.
There are apps for reading books on computers, but for most people, that is a supplement.