To add to Rookies comments.

The data in data warehouse is humongous, trillions of records which are maintained... which means if you have more joins the execution of queries takes more time.

When the data from the OLTP system is migrated to the warehouse the data is categorised and confirmed. The data source of warehouse could be mutilple. the data needs to be cleaned, processed and fixed at various levels.

there are various data marts present in warehouse which are dependent on each other . Thus it is recommended to have less joins. this sometimes leads to duplication of data which is fine and in turn gives better performance .