Mostly if you see the internet only up to 3rd or 4th normal forms are discussed, if you read there are up to 7 normal forms so why aren't they discussed by people on the internet you hardly find any example related to them whether on YouTube or any any other place.Answer1:
Mostly a combination of 1. ignorance of benefits and 2. limitations of DBMSs.
One could blame the limitations on the ignorance--of designers & implementers but maybe mostly of users, leading to no market, leading to limitations. Ignorance includes not just the benefits of normalization but of integrity constraints.
All base tables can be normalized to 5NF. However, that might not preserve all functional dependencies. Preserved ones get enforced by key constraints (that certain column sets are UNIQUE NOT NULL) while non-preserved ones require equality dependency constraints (that the set of subrow values in two column sets are always equal).
The DBMS limitation issue is that declarative constraints necessary for 5NF integrity are not usually available, so people don't normalize that far. But even so, if they only want to use the declarative constraints typically available (PK, UNIQUE NOT NULL, FK & CHECK) then they ought to normalize to 5NF and then consciously denormalize, at the cost of multiple table update transactions to deal with the consequent update anomalies.
PS Normal forms are just different conditions that have been found with helpful properties. Also different axioms and algorithms. The normal forms on the continuum from 1NF to 6NF are those dealing with problematic FDs (functional dependencies) and JDs (join dependencies). They can be ordered so that if a relation value or variable satisfies a form then it satisfies the forms before but not necessarily after. Currently: 1NF, 2NF, 3NF, EKNF, BCNF, 4NF, ETNF, RFNF aka KCNF, SKNF, 5NF aka PJ/NF, Overstrong PJ/NF, 6NF.