Because picking any other number is somewhat arbitrary, and shifts the curve such that it becomes the new "average".
1500 is selected not because we assume everyone coming in is average, it is that we have 0% confidence we know their MMR. This is true, we have no data on them yet. This lands them as potentially being anywhere on the bell curve, the average place of the bell curve is 1500, so they go there.
We can move them to 1200 for example, but over time this largely just changes 1200 to be the new average. Players who are really 1500 today would see themselves over time shift to 1200.
The second issue listed there is initial placement fluctuations. Since we have no confidence in their MMR, we test them. The "quickest" way to test them is to move them around a lot and heavily, overshooting and undershooting their real MMR until we can narrow it down.
This is the fastest method of placing everyone and finding their actual MMR, but it has the downside as you have suggested. On average this works amazingly well, but looking at individual cases it is possible for someone to be carried too high. They 100% won't stay there, it is impossible to remain that "lucky" but when looking at only 3-5 matches, it is totally possible.
You can slow this down, which will mean players will move slower and the extremes become less extreme, but this does mean we are essentially saying players will be at the incorrect MMR for a longer period of time.
So you CAN change starting MMR and you CAN slow this effect down, they have their own issues and impact the Matchmaker in non-trivial ways. Match quality will go down across the board.
We are discussing ways to address this but it is worth going over why things are the way they currently are. Matchmaking is often a matter of tradeoffs, what experience do your prioritize? Matchmaking isn't responsible only for making even matches, but ranking players in a way that can be used to make those matches. Making "more even matches" will often come at the cost of not testing players Rank accurately.
If I argued moving it just makes Silver ELO Hell, I probably just conceded the idea of ELO Hell for the sake of discussing why moving what a Division's MMR range is associated with won't change your experience; but I would contend that ELO Hell doesn't exist. There are many reasons for this, but if you are stuck at an MMR and you have played a significant number of games to wash out natural variations (50-100 games) you belong at that MMR.