If we live in a virtual reality, then why do we never see people or objects appear or disappear suddenly? Is this proof that we do not exist in a simulation?
No. We could be either real, or living in a well-programmed simulation, or unable to notice glitches (since this is the world we live in; eg: do you think a Sim notices pixelation?).
Doesn't prove anything.
How do you know that things don't disappear? You've never misplaced anything? Just because the control program has noticed you searching for something; thus runs a check on items, finds that one has been lost, then (sometimes!) places the item in an out-of-the-way spot that you haven't checked - or have already checked once or twice! I think this would be the definition of a disappearing item in the context of a program of simulated reality.
Existence does not have to be "real". Reality is a moot point. There is no sufficient assertion what reality is or should be. We authenticate ourselves. We do not need an authority figure to do that for us. If one wants to exist, yes, go ahead. If one wants to debase self and not exist, the same applies.
The question mentions "people or objects appear or disappear suddenly". That is quantum mechanics, which simply means we do not have any empirical evidence yet to make any logical assertions on that level. We will have that in a few years, then we will have a new quantum level which will represent what we have not come to understand yet.