Magento choose not to use a frontend framework such as bootstrap for a good reason, frontend technologies like bootstrap come and go and what is relevant now may not be relevant in say 5 years time. Developers don't want to be stuck using old technology because it is a fundamental part of the system and a requirement. Instead they implemented their own UI component library which, put basically, consists of some styling for a bunch of UI patterns commonly used on ecommerce websites, it is based on Less. These components are used extensively in the default luma theme.
It is probably worth noting too that Magento has recently announced that due to feedback from the frontend developer community Magento 2 will be moving from Less to Sass. This means that in future versions of Magento the UI Component Library will likely be based on Sass rather than Less.