You COULD send them less often, but:-
1) If one fails on launch or landing, then you have nothing for a long time
2) Mission concepts are numerous and new ideas and landing sites are constantly evolving, so there are always many more places to visit than even a frequent mission timeline
3) Publicity and public awareness is vital to support of space programs and a "ho hum, the SAME rover is STILL driving around the SAME bit of Mars after 5 years" is NOT a big seller, IMHO...