From an outsiders perspective I find these calls of impatience pretty bewildering.
If anything Solskjaers status as a club legend and figure of one of the best moments of the clubs history has given him more leeway and benefit of the doubt than most managers would have enjoyed. There is a certain romantic notion at play here that a lot of people would love seeing a part of the class of 92 being the person that lifts the club back into the heights of the Ferguson era. That is understandable.
However, stepping away from this viewpoint and looking at it objectively, here is where Solskjaer stands:
This is his third season as manager and he has been given vast amounts of ressources to be successful: around 300 Mil. € net spend over the last three Summers on top of generous signing fees and one of the highest wage bills in club football speak a very clear language. Manchester United is in terms of financial capabilites, prestige and reach without a doubt an elite club and with that comes a simple truth: Elite clubs constantly challenge for titles.
Does Solskjaer do this?
Do people believe that his United is capable of going toe to toe with Guardiolas City, Klopps Liverpool and Tuchels Chelsea over a full season? A lot of people don´t because while the results have not been terrible, United certainly does not play like a serious title challenger and still sits behind all three in the table despite having a far more favourable schedule (all three beforementioned clubs have played and taken points off each other at this point) which should have given them the opportunity to actually build up pressure on the opposition. If they don´t believe in Solskjaer being able to win the EPL they are right to question his position as that should be the baseline for a United manager.
The same applies to the CL. By most measures United should be in the conversation of winning the whole thing, not having to worry of making it to the KO stages.
It honestly boils down that Solskjaer has not proven to be able to rise to the standart a manager of a club of such a size should be able to do. Neither before he took the job nor in his time at the Old Trafford.