Why cannot we merge one to one relationship entities in one column?

ok lets think of a db design that has a one two one relation ship

maybe table_1 has column firstname and table_2 could have column gender is not consider as one to one relationship becuase in a table gender one gender could have assign to any person.

So lets start with the example of table_1 has a column of firstname
table_2 has a column student id this is an example of one to one relationship because not all student has a the same school id, however this design seems so pointless becuase you could join this column to table_1 instead of separating it the different table..