Because covid is real.

There's 2 big things about server congestion.
The first is that XIV saw a massive increase of players during covid.
The second is the semi-conductor shortage, it already existed before because of crypto, but it's now even worse due to reduced supplies. Demand increased, supply decreased.

FF XVI delay was explained in the letter.

Yes, XIV is far from perfect and has many, many bumps and rough spot.
But covid complication is real and even vaccinated + masks, it slows down everyone. Especially the semi-conductor supply that is not in Yoshida's control.