Because school days begin and end and lessons and units and school years do too, any learning done in school has to as well.
Or their peers or an authentic audience or 'client' but rather the external and singular (e.g., the teacher, a computer algorithm, etc.) This 'quality judge' is usually a person (i.e., teacher) and singular and often not an 'expert' or able to give the time and care and affection a topic or skill or project deserves due to extraordinary time demands made on them elsewhere.
In school, the source for the standards for quality of work are often external and almost always 'singular' (come from one person). Among other effects, this commands the depth, quantity, timing, and 'framing' of the learning feedback.
And done so by the above 'quality judge.' This means it is rarely based on curiosity, genius, or intended application.
Unless it's in one of the types of blended learning or project-based learning somehow done in the 'real world,' school learning is done in school. Even if it's not, the 'walled garden' of learning is institutionally-centered and standards-driven rather than based on curiosity, buying/selling, creativity, or some other 'more authentic' context.
This one isn't the teacher's fault-it's just inherent to an outcomes-based, teacher-led classroom. It's not only difficult for a teacher to teach something they don't know or aren't comfortable with, but it can also reduce student learning or even be dangerous (e.g., privacy concerns in social media, for example).
In the classroom, culture is often 'added in.' In the real world, culture is (generally) often the foundation or focus of the learning and learning process. Gender, race, poverty, sexuality, politics, dance, art, and so on are rarely the driving force behind learning while in the real world, obviously they are.
While learning in the real world can sometimes be based on 'math' and 'language arts,' more commonly those content areas are simply sources to inform and improve the learning process itself. For example, if someone wants to learn to build a fence, they might learn to calculate area and perimeter to accomplish that goal. In school, those content areas are 'mastered' in hopes that one day students might need to use that skill or apply that knowledge.
And those grades-rather than the work and process and products and networks created while doing that graded work-tends to 'stick' with students.