Because the characteristics that enable a dangerous game rifle to perform best, are missing from most factory rifles. This can be as mundane as shortening the rifle's LOP, or adjusting the trigger. It could be installing a barrel band front sight with a rectangular post and a ghost ring rear sight, or if you prefer express sights. It could involve a drop box magazine to increase a Model 70's appetite. It might involve installing a finger deflector behind the trigger guard and moving the front sling swivel attachment to the radius of the forend tip. Milling receivers to accept stripper clips is a nice touch, although I haven't done this to mine, it makes the use of a forward mounted scope essential, and the installation of a quarter rib can facilitate this. Some rifles benefit from changing the angle of the bolt handle to prevent bruised knuckles, or just to improve cycling. Some rifles would benefit from a second recoil lug. Some rifles would benefit from a glass bedded bolt that extends from the butt, through the wrist of the stock to prevent stock failure. All rifles benefit from a good bedding job, and in the case of a dangerous game rifle with enthusiastic recoil, a bedding job will prevent a split stock. If the barrel is shortened, counter weights can be added to the forend to maintain balance.
Bolt action rifles provide the best balance between cost and performance. Double rifles are expensive British double rifles are cripplingly expensive. I've fired a .458 BAR that belonged to Pounder, but I wouldn't want one. After carrying single shot rifles for protection, I've come to the conclusion that they aren't particularly well suited for it. With the exception of the 95 and 71 Winchesters, few lever actions are well suited for the cartridges that are normally associated with dangerous game, and that goes double for pump actions.
Perhaps we are using terminology differently, but to me you just described a grocery list of modifications to be made to a production rifle in order to make it more suited to personal taste. Thats a bit different than a custom gun which is generally built from scratch to the requirements of the owner, which could certainly include alot of the features you just described.
The OP seemed clearly interested in CUSTOM guns, and not simply modified factory guns. I am just curious as to why?
Lastly, since I have never been on a dangerous game hunt, help me understanding why some of those modifications are necessary.
I fully understand anything that helps with recoil management. Likewise anything that improves fit for the shooter, like LOP. These things aren't unique to big cartridges, although obviously the recoil becomes more important when there is alot more of it.
Front Site on the barrel band? Whats the theory there?
Finger Deflector? I have never heard of it.
Magazine. I like mags, but would you say they are essential for DG hunting? If so why?
Moving front sling swivel?
Stripper clips. Is that in addition to the magazine or in lieu of? Does big bore ammo come on stripper clips?
Moving the scope forward to permit stripper clips would necessitate a scope with pretty long eye relief. Typically you only see stripper clip read receivers on guns with open/iron sights.
You mention protection, and that was my second question. Are we hunting, or defending. If hunting, then I think magazine capacity and reloading should take on less of an importance. Long guns aren't well suited for short ranges. If defense is the concern, and ammo capacity is desireable, then shouldn't we be taking about a semi auto?
Educate me.
