Base Rasengan in itself cannot be thrown, however some improvements and mastery can allow user to throw as projectile. Base Rasengan is a spherical orb of Chakra and indeed enough chakra is invested just to contain its form, rotation and power and if it was to be thrown, then its form and power would be diminished by the time it would reach its enemy (it would also be assumed that its trajectory would be way off that the user wanted).
Naruto initially used Rasengan as his go-to jutsu but he realized it can be vastly improved by adding Chakra nature. However, he struggles to combine the Rasengan with his wind-nature, which he describes as needing to look left and right at the same time. He is eventually able to overcome this obstacle by using a shadow clone to provide the nature transformation while he and another shadow clone form the Rasengan, thereby creating the Wind Release: Rasengan (which can be thrown). By further shaping the Wind Release: Rasengan, Naruto creates the Wind Release: Rasenshuriken (which can also be thrown).