Hello @asha23 and @peppir,
It’s not about only Dokan plugin, most of the plugin save their table in the database because if you re-install the plugin later that time you will get your data back. Even WooCommerce does this as same. Please install WooCommerce and then delete it, you will find that order table and all other WooCommerce related tables will be available in your database. So, I think you did not get the correct point.
If we delete all the data from database after deactivating or deleting the plugin then you would not get any data when you will re-install the plugin. Someone may want to re-install the plugin so, we have to think logically for everyone.
Thanks!