>

Not everyone believes in utilitarianism, whereby measurable harm to a living person is the only thing that should be avoided.
Even under that model though:
The idea someone might desecrate or otherwise disrespect my grave, (or worse, those of those I care about) would distress me greatly. I don't think that's an uncommon sentiment. The only way to avoid this distress, is to respect the already dead, so as to assure the living that they and their loved ones will be respected.
If you think the wishes of the dead are irrelevant, do you think we should abolish wills and ignore any promises we make to the dying? I think most people feel there's at least some moral imperative to respect people's wishes even after they're gone.
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There is endless education to be had, from diet to how people died, what medical procedures were done at the time, what diseases, etc., etc.
Also, in the case of something like Egyptian tombs, esp the ones of the wealthier or more famous -- looters have been looking for and looting those for centuries. 
Isn't it better to find, excavate, and treat respectfully than leave or replace and have stuff looted? >

Looting! I didn’t even think of that. Excellent point. I think that’s definitely changed a lot of my perspective. Because even if I disagree with the general concept, it would probably happen inevitably with how greedy people are ∆ !! >

There is this moment when public, political or scientific interest outweighs the right of a dead person to remain in their burial place. This is true for modern as well as for ancient graves. Basically, if there is anything significant to learn from your corpse that may not be learned otherwise, and that is of importance to currently living people, your corpse may be exhumed. Or if you happen to be buried under circumstances that are subject to examinations and maybe even criminal proceedings (like if your corpse is buried in a mass grave behind a catholic boarding school or sth of the sort).
The thing is, the further you go back in time the fewer graves are left, and the more can be learned from those that are still there. Not just from the remains, but also from the surroundings, because graves are one of the best place to learn something about a society by looking at how they treated their dead. Graves sometimes contain well preserved funerary goods like fabric, food, medicine, jewellery - stuff that otherwise didn't survive the ages and can basically only be found in well preserved, undisturbed grave sites. 
In many cases, the alternative to not excavating also would be to destroy the site or letting it get destroyed, as in our modern days many sites are found by accident. By that I mean building sites for roads, houses etc. Or the sites are in danger to be affected by climate change (thawing, flooding,...) and generally impacted by human activity (landscaping, agriculture, deforestation...), not to mention looting. >

I would argue it isn't immoral to dig up any grave. After all, who is it hurting to dig it up? Sure, it would cause distress to some people if you dug up their recently deceased ancestors, but besides that, what's the harm? It's not like the dead give a damn.