(1) This is just because of Intel's naming system. There are only i3, i5, and i7. It is like naming people. There is no reason, it is just an artificial setting by the namer (company).
(2) Intel’s current layout in the home CPU market includes the entry-level “Celeron” series (such as G1840), the advanced “Pentium” series (such as G3240), and the high-end i series, and Core i is divided into three grades: low, medium and high. i3 is the entry-level performance level, i5 is the mainstream performance level, and i7 is the high-end fever performance level.
(3) Naming philosophy (metaphysics)
There is a saying that: 3 5 7 are all prime numbers (prime numbers), which is the preference of Intel's decision-makers. Interestingly, Intel’s competitor AMD chose A4, A6, A8, and A10 when naming its APU family. The two competitors have a tacit understanding of the intersection of numerical naming.
There are many cases of naming according to this interval number, such as BMW's 7, 5, 3 series, steak's 3, 5, 7 rare, etc.