There are actually a number of cross border bus services in Kingston and Epsom area, some are TfL (216, 293, 406, 411, 418, 465, K3 etc)
Surrey also has some subsided services into these big towns, although some are commercial. Many have received new buses due to TFLs Ultra low emission zone for buses, coaches and trucks from October next year
Probably because of the strange geographic boundaries, goes back about 50 years when Epsom, Ewell, Esher didn’t become London boroughs but Kingston and Surbiton did (as a combined Kingston). Pretty obvious when you look at map. Of course the M25 (which didn’t exist then) would be more sensible boundary nowadays