Expecting employers to always get back to is unrealistic (in my opinion).
Some of them use "We'll let you know" in exactly the same way as it's always been used at the end of auditions for actors. (i.e. it really means, "You've not a cat in hell's chance of getting the role, matey"). Others might genuinely mean to get back to candidates but then be too busy to do so (or they might simply forget).
Over a two-year period, I applied for over 2000 jobs. Out of those, only about 110 even bothered to acknowledgement my applications. Of those 110, only 3 gave me interviews.
Of those three, the first employer got me to do a day's unpaid work trial (with a 13 hour shift that had hardly any real breaks in it) and then never got back to me. (My attempts to contact the company were ignored).
The second interviewer told me that I'd got the job subject to the approval of the 'big boss' when he returned from holiday but then I heard nothing from the company. When I phoned them, they said that the boss had decided to restructure job roles, meaning that there was no longer a vacancy.
The third employer seemed impressed with my interview and offered me a day's unpaid work trial. Again he seemed very impressed and asked me to go in the following day (with pay) to complete some tasks that I'd started for him. At the end of that second day he seemed to be even more impressed and I thought that I'd probably got the job. However I then heard nothing from the employer until I wrote to him, asking to be paid for that second day. He sent me a cheque, together with a letter stating that he couldn't employ me as I was 'too intelligent'!
So expecting to hear back from every job you apply for, even if you've had an interview, is simply unrealistic.