Why use a quasi experimental design when you can do an experiment? I read about quasi-experimental design and the variations it has such as pre-test post-test non-equivalent groups, and also experimental designs. Having compared these two, I ascertained that the difference between this variation of quasi-experimental design(pre-test post-test non-equivalent groups) and experimental design is randomization.
However, what I couldn't understand was:

Why should quasi-experimental design  even exist when we can do randomization easily and the randomization has more benefits than matching the participants?

Honestly, I search for the answer but I couldn't find any, and obviously 
that is why I'm asking a question, yet if you find any alike question I would be happy if you refer me to it.
I genuinely appreciate it if you refer me to at least one resource as well.
The reference book for the discrimination between these two designs:
Introduction to Research Methods and Data Analysis in Psychology, Third Edition by Darren Langdridge and Gareth Hagger-Johnson
The first paper I found searching for "advantages of quasi-experimental design" was this one:

Schweizer, M. L., Braun, B. I., &amp; Milstone, A. M. (2016). Research methods in healthcare epidemiology and antimicrobial stewardship—quasi-experimental designs. Infection control &amp; hospital epidemiology, 37(10), 1135-1140.

Most of the benefits are pragmatic: less cost, potential for retrospective analysis, bypassing ethical considerations that present barriers to randomized trials, etc.

An example of the type of quasi-randomized design I've often been involved in myself is when the data have already been collected in normal medical practice. Sometimes these patients are difficult if not impossible to consent: an intensive-care unit population, for example. Those patients are very sick and their imminent needs are live-saving care; there may not be time to obtain consent for a randomized trial. However, if practices and policies change over time or are different in different facilities, you can use a quasi-experimental design to determine whether those changes or differences in standard of care have an important impact or not.