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.
Players and coaches have quite striking differences, players have to be smart in digesting every instruction from the coach and applying it on the field, while their coach has to think about having a strategy and it can be different based on the strength of the opponent they will face. Coaches must also have the ability where they must know very well the characteristics of each player they have, including what their playing style is like.
Not everyone who is successful as a player will also be successful as a coach, we have seen many examples in this case and also not all coaches have to be born from players, many coaches who even started out as translators can become great coaches, like Jose Mourinho for example. I don't want to compare which one is more difficult, because both are different things with their respective roles. However, maybe someone has different abilities in different fields, in this case coaches and players.