Because longbows didn't have similar rates of fire, nor similar range, nor similar penetration to 20th century rifles. They neither had a similar level of ease of use, a similar amount of maneuverability, nor were they similarly easy to shoot for prolonged periods of time.

Longbows required a tremendous amount of physical exertion to fire at any relevant rate of fire. They required a steady stream of arrows -- thousands upon thousands for large forces -- and they are not easily carried on the move nor are they particularly light en masse. They were notoriously inaccurate as well beyond a relatively short range compared to rifles. They were area denial weapons first and foremost, not precision targeted weapons. Think of them functioning more in the war of supporting fire, ala mortar fire or close artillery, than frontline riflemen.

It's still a contrived example but any attempt to compare warfare in 1400 to warfare in 1940 is going to be inherently, to be frank, impossible and contrived. War in the medieval and early modern period wasn't about small squads taking land because small squads of men did not have a comparative amount of firepower as a 1940's one. Sizes of tactically independent forces did not decrease because people in the past were dumb, they decreased because as time increased the amount of firepower a smaller group had to decimate a larger one would increase. It became too clunky to operate in terms of 1000 men and more in 250, and then 80-100, and then 15 and so on; that all correlates with an increase of relative firepower. Longbowmen in the medieval period did not have that type of firepower.

If they were on their own operating like that, they would simply be cut down by a larger more cohesive force of cavalry or overwhelmed in sheer firepower by a larger group of bowmen. Further bowmen lacked any capacity to 'hold their own'. There's a reason that the 1500's and 1600's were rife with 'pike & shot' formations -- bowmen and early muskets could not match a large group of guys with pikes and swords coming up and simply forcing you to leave. But yet ranged support still had its obvious uses. Once we reach the 1700's though and especially the later parts of the 1800's, it becomes impossible to just charge up and take a position by using spears, or, later, bayonets. Thus the transition to musketry as a whole. Honestly the image in my head is just funny -- what are the Welshmen 'covering' eachother from? 500 men charging at them and ready to cut them down? "Cover" was just not a 'thing' in the period of mass formation warfare as staying a cohesive force that could output coordinated fire was more important than anything else.

If you'd like a good book on warfare in this transition period, check out H.W. Koch's The Rise of Modern Warfare: 1600-1815