Why MMICs are surface-mount microwave chips, and the interconnects on the circuit board must be controlled-impedance transmission lines (usually 50 ohms) right up to the package leads. Microstrip does that with just a trace of calculated width on a dielectric board over a solid ground plane, so it is easy to fabricate as part of the PCB artwork. Its dimensions and the board's dielectric constant set the characteristic impedance, letting the designer match the MMIC input and output without extra hardware.
Watch out Miniature coax is the tempting answer since it is a controlled-impedance line too, but you cannot solder coax to chip-scale leads without launching problems; coax and waveguide are for getting signals between assemblies, not between devices on a board.
MMIC sits on a strip of board: microstrip. Impedance comes from trace width over the ground plane.