700 word discussion: why is it more common to work with models “modded off-the-shelf” (MOTS) or even tailor-made models to suit specific questions and conditions?

Chapter 10  – Policymakers are tasked with making decisions on issues characterized as wicked problems because of controversies, unknown relationships between causes and consequences, and (consequently) uncertain futures. From this perspective, it would be desirable to map the decisions and their possible outcomes prior to the actual decision making because that would generate certainty in ambiguous situations. Broadly speaking, this provides the motive for using computational modeling for policymaking as expressed in, e.g., policy informatics. 

Q1:   From this revelation, why is it more common to work with models “modded off-the-shelf” (MOTS) or even tailor-made models to suit specific questions and conditions? 

why is it more common to work with MOTS as indicated above? provide a clear narrative to support your responses.