So many devs tell me "I'm not ready for a mid level role yet" or equivalent, even when thay _clearly_ are...There's a difference between being ready for a midlevel role, in the sense of being able to take on the responsibilities and requirements, and wanting to take on the midlevel role. For example, if a developer sees that the "seniors" on his team 1) don't get to program as much, because they're in meetings all the time 2) have a lot more stress, because of having to take responsibility for design decisions 3) don't get compensated very much more and 4) sacrifice their ability to switch jobs easily by becoming more specialized in the methods and practices of the company they're currently working for, I think it's entirely rational for the developer to not desire that promotion.
But, of course, it's not politic to say, "Oh, I don't want that promotion, because it's a lot of work and I don't see any upside to it," so people make euphemisms. Saying, "I'm not sure I'm ready for that," is a good way of demurring while not causing offense, and simultaneously leaving the door open so that they can be considered for a promotion in the future.
The next time someone says that they're not "ready" for a mid-level role, I would like you to ask yourself, "Are they not ready, or are they not willing?"