The reason why Sony have not launched a streaming service is pretty simple: they do not need to in order to maintain current revenue levels long term. Disney, NBCU, WBD, and Paramount make up a significant portion of their overall revenue from broadcast and cable services. That industry is in free fall thanks to Netflix and Amazon launching low cost, popular, streaming services. In order to chase that lost revenue the likes of Disney, NBCU, Paramount, and WBD have had to launch their own streamers. The problem is that is super expensive and needs heavy investment in the early years to rollout worldwide and to grow to scale. It will eventually be profitable for most of them.

Sony have not got the content to be truly competitive in an overly crowded streaming industry. They were always the weakest of the big studios. They also do not need to invest in streaming as they have very little revenue tied to the collapsing cable and broadcast industry. Sony do not need streaming because they did not have cable and broadcast businesses of note in most markets.

They did invest in niche anime streaming service Crunchyroll which looks an excellent pickup at the $1 billion they paid for it.