Why Primary Key constraint creates Clustered Index on the PK column by default?
That is what MS-SQL-Server programmers decided the default to be. A good clustered index is one that has unique values (as the Primary Key), is narrow (as most primary keys are or at least should be) and is ever-increasing. So, most of the times, the primary key is a good (or the best) choice for the clustered key (there can be at most one clustered key per table).
Can we create a table which has a primary key, but NO clustered index?
Yes, you can. By explicitely defining all indices and especially the primary key as non-clustered. If you think that you don't need a clustered key on a table, you can do that and have the primary key as non-clustered. The unique and not null constraints will still be enforced.