Of course not all great players make a good coach but in most cases people believe that since a player was very good during the time he was playing he will actually have a good quality of a coach forgotten that there is no how they can compare those coaches that follow the right process, so actually in the case of Lampard indeed he was a good player during his time but that wasn't enough to immediately go into coaching without much of a coaching experience.
Being a good player is not a criteria to becoming a good coach. In fact, there's a trend of most average players who turn out to be great managers of their time. The likes of Sir Alex Ferguson, Pep Guardiola, Conte, Arsene Wenger where not really talked about as top players during their professional careers but performed exceptionally well in their managerial careers. Interesting enough, there are players like Thierry Henry, Paul Scholes, Rooney, Gerrard and the likes of Lampard that we are talking about. Seasoned managers like Jose Mourinho never really played professionally as players yet he recorded great milestones during his active years.