Actually it is because it is a different problem they are trying to solve.
What Signal is solving by these additional proxies is to avoid being blocked. So this is orthogonal to avoiding the detection of users.
The real way to avoid detection of users is going through something like Tor.
Edit 2:
The real problem is that, in countries where Signal is blocked, it is ALSO forbidden and illegal.
If it was just blocked and not forbidden, nothing wrong in working around the blocking.
But actually permitting users to work around the blocking when it is illegal is not helping them, unless there is also a way to hide them. Else it helps them commit (overtly) the crime for which they risk many troubles.
Exactly, according to NGO's people get lashed and jailed for online activities. After Signal has been blocked being detected could actually endanger peoples life.
They could have prioritized the dev (or scheduled the release/deployment):
1/ integrate some Tor-like system, and 2/ the amateur proxies feature
Actually they can still do it, by deactivating and putting the amateur proxies feature in standby temporarily while the Tor-like system is being implemented.
The very least they can do is not denying that, now they have already deployed 2/, developing 1/ is becoming an emergency.
It does not cross their mind that the users are immediately endangered too.
They don't understand that it is very easy to identify the proxy users once the Signal proxies themselves are detected?
I'm here replying on the top level to this comment, because I think this is very important: https://news.ycombinator.com/item?id=26076113
Edit:
Actually it is because it is a different problem they are trying to solve.
What Signal is solving by these additional proxies is to avoid being blocked. So this is orthogonal to avoiding the detection of users.
The real way to avoid detection of users is going through something like Tor.
Edit 2:
The real problem is that, in countries where Signal is blocked, it is ALSO forbidden and illegal.
If it was just blocked and not forbidden, nothing wrong in working around the blocking.
But actually permitting users to work around the blocking when it is illegal is not helping them, unless there is also a way to hide them. Else it helps them commit (overtly) the crime for which they risk many troubles.