Why not safe guard yourself and have more than 1? You never know, they might impose a tax in the future if you hold more than a certain amount. It is the future so who knows but always prepare some kind of trust fund for a rainy day is my motto and it has served me well in any financial situation I have faced so far.
Because its not a certainty and it is possible for BTC to have no value. Its fairly certain gold will go up in value, why not have alot of that; the main reason is gold is not a useful product (by itself) and literally it can weigh you down if price falls its a liability.
The same question faced people in the housing boom, if 1 house will rise in value then why do you only own 1 house, why not 2, 3 or more. Really its the same answer, you only need 1 house and prices can fall for 10 years maybe and you have the problem of paying bills on all those things you dont need or use meanwhile. Of course people buy too much, then sell altogether at a loss which is a main cause of longer low prices and price spikes.
Basic conclusion is dont over invest in anything and definetly not purely bitcoin alone, not many of us absolutely require to have alot so its just speculation or gambling. If you are already rich then sure buy a few as pocket change for a rainy day, compound interest on mortage debt often makes that most compelling for average people