Why don't some EPSG have proj4?

[المشرف العام]

I was looking into PostGIS and its defined Spatial Reference Systems, and I found that some have no proj4.
Why don't they?

SELECT auth_name || ':' || auth_srid, proj4text FROM spatial_ref_sys ORDER BY proj4text

"EPSG:22300";""

"EPSG:29701";""

"EPSG:3577";"+proj=aea +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=132 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "

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