Exactly why the time taken by 'memory reads', is not considered while calculating time complexity of an algorithm? I am sorry if I sound repetitive(I've asked a similar sounding question earlier - Relevance of memory reads while calculating the time complexity of an algorithm). Actually, it didn't answered my intended query. May be I framed the question inappropriately, and in an unnecessarily complex and indirect way.

In the current/present question, I am referring to the magnetic disk memory as it definitely takes much time, but if the primary memory(RAM) also does take much time relative to processing, then it can too serve the purpose here. 

Also, I am specially concerned here with the algorithms where number of memory reads are some function of 'n', instead of being nearly constant(because in latter case, this question is not applicable or doesn't make sense anyway).
People don't usually refer to the hard drive as "memory" though the term "external memory" is common in this area of the literature. As far as disk accesses are concerned, they are taken into account in the analysis of algorithms that are explicitly written to access disk. Some of the more popular models are the two-level external memory model, the parallel disk model, and the cache-oblivious model. There are also hierarchical memory models. Algorithms and Data Structures for External Memory particularly chapter 2 describes the parallel disk model in detail, briefly describes other external memory models, and provides pointers into the literature. Some of these ideas and results go back about half a century.