While I couldn't find quotes directly from song writers, there have been people who have wondered the same thing and have come up with a good analysis. I'll list up some reasons, in no particular order.
It's is easier to identify with ぼく
Many songs are ambiguous as to the gender of the singer; they can go either way. In these cases ぼく is selected often. It is easier to resonate with male fans and they can sing the song in karaoke with more emotion due to that. わたし actually sounds more feminine in terms of lyrics, so by using ぼく it makes it easy for the fans to insert themselves as the singer.
Note that songs strictly from a female perspective would likely choose わたし as there is no need to put the male listener in the singer's shoes.
It gives the lyrics another layer of depth
By making a song "non-gender", the lyrics can have double meanings.
This blog analyzed lyrics from Nogizaka46's 君の名は希望. Take this line:
While this song can be read as a love song, this can also be interpreted as the idols themselves talking to the fan, or even the fan to the idol. The "idol" wants the "fan" to look at her. The "fan" want the "idol" to see him. Using わたし here would limit it to one way: only the idol to the fan. It's no longer a two-way street. These kinds of reciprocating lyrics are common in idol songs and it's most likely because it resonate with the fans.
ぼく could be me (the singer), me (the fan), me (the lover), me (the friend), etc.
It is the right level of youth
Sometimes it may seem わたし may feel just as appropriate: it's a song that is obviously from the perspective of a girl. But わたし may sound too mature; ぼく gives off this feeling of youth, of innocence, that the lyricist is looking for. It feels nostalgic, like reminiscing of one's youth.
It's easier to fit into the melody
ぼく is two moras, わたし is three. It is much easier to fit the shorter word into the melody. Note キミ and あなた follow that same pattern. Take the songs you listed and imagine how it would sound like if they were replaced by わたし.
A lot of songs, especially for idols, would be made-to-order, with the composer and the lyricist being two different people. In those cases, the melody tends to come first, and the lyrics are then created and added. This means the lyrics are limited what the melody allows, so every space counts.
This blog makes a good point and shows that female singer-songwriters actually do use わたし. This is because most of their songs are from their point of view. They can do this they are able to make the lyrics before making the song, and even if they decide to use わたし later on, they have the ability to arrange the song appropriately.
It's a current trend
This blog takes some samples (though this includes male artists) from different years, and we can see that towards 2000 ぼく becomes prevalent. This is around the same time the modern-day idols, like AKB48 appears (and they use ぼく a lot).
Looking at the older periods, we can see that わたし・あなた were more prevalent back in the day. So another 20 years down the line, we might see another shift in first person pronouns used in songs.