Because it restricts freedom and is consequently life-denying. I oppose it on the same grounds and for the same reason that I oppose slavery, racism and patriarchy. But that is not something I would expect a proud and unrepentant colonizer to understand, so it is little surprise that you are confused.
This is the exact difference -
You are not forced for hard labour, your movement and other rights are not restricted compared to whay you'd have in a republican alternative.
You are not racially segregated, and actually not at all segregated based on any characteristic or yours.
And you are not bent down to fulfil the wishes of your patron or lord.
So why choosing these comparisons?
It seems like you resent the essense of the high position of the dominant side, and not necessarily the suffering that it causes to the oppressed side.
Republics can very well be troublsome in all of those fields. Republics can take your civil rights, enslave you, and racially segregate you. The UK of our times is a good example of a monarchy (well..) that is almost flawless in these regards.
In a metaphore to another field of political division - you would like to limit the rich regardless of wether it helps the poor or anybody else.
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Allow me to partially ignore that last stupid comment you added down there, and just remind you that my initial post stated that I have nothing for the monarchy at all.
I just think that some people have, thus royalty can be a good thing for a country like the UK to have. In a nation-wide view.