Why At microwave frequencies a wavelength is only centimeters long, so even a half-inch lead is an appreciable fraction of a wavelength. Signals take measurable time to travel that distance, which shows up as phase shift (and series inductance) between parts of the circuit. Keeping connections physically short keeps their electrical length, and therefore the phase shift, small so feedback paths, matching networks and oscillators behave as designed.
Watch out Noise figure is dominated by the first-stage device and its input matching, not by lead length; the neutralizing-resistance and compensating-capacitance choices are invented terms that don't describe why microwave leads are kept short.
At 10 GHz a wavelength is 3 cm, so an inch of wire is most of a wavelength. Short leads = small phase shift.