You are right; probably I have to state the question in a more precise way, which would be: why is it such that this model seems only to be used in huge "enterprisey" contexts? The model of course depends on a certain modularity of the software in question, but hasn't that been a core concept not just for the biggest players, but also the smaller shops for a while now?
Implementation costs a lot, not just in coding, but in all the extra work that goes into sales, gathering requirements, training etc. Whoever nominally pays for all that work, it translates into a high price tag for the customer. Only large enterprises will get enough value from the customisations for that high price tag to be worthwhile.
All else being equal, a smaller company will spend a lower absolute amount to work around imperfections, and get lower value from any product overall. So of course they will not be willing to pay as high a price. Hence, for commodity use cases, use commodity software rather than fully/semi-customised.
