Why does bulb mode usually require holding the shutter button, and where does the term “bulb” come from?

In days of old, the shutter release was pneumatically triggered from an air bulb. You squeeze the bulb for as long as you want the exposure.

From The Focal Encyclopedia of Photography:

When lens shutters were introduced in the 1880s, one way of tripping the mechanism was by squeezing a rubber bulb that was connected, by a long rubber tube, to a small piston. One option for exposure was duration, called instantaneous. The other exposure was keeping the shutter open as long as the bulb was compressed and was therefore known as a "bulb" exposure.

It looked something like this:

And so we come to the modern world and the name has stuck. There are other mechanisms for doing a bulb exposure that doesn't mean squeezing something. I personally like mechanical cable releases that look like:

Though when you loose the threaded release as the way to hold it down, you find yourself needing some other approach to do the duration of the exposure. You can hold the button down (the way that it worked before too - just the cable release made it easier).

Holding down the shutter button is the surest way to tell the camera how long to do the exposure. It has worked well from the days of the first in lens shutters until relatively recently as everything is now electronic or wireless (though when you lack those, it is still useable).

You can find electronic ones now days. For example:

With this release, you can hold it down, or you can push the cover to lock it down. Canon (for a while, I'm not sure if this is still he case) could make use of a DIY remote for which you could make your own way to 'hold' for the duration of an exposure.

Wireless releases such as the ML-L3 tend to use a dual press (holding the button down has too much possibility for error if the signal is lost):

When the ML-L3 remote control is used in M mode, users can select '- -' as the shutter speed. At this setting, the shutter opens when the shutter-release button on the ML-L3 remote control is pressed (2 seconds after the button is pressed in delayed remote mode) and remains open until the remote-control shutter-release button is pressed a second time (maximum exposure time is 30 minutes).

Going back to the world of large format, you can see the 'B' setting (bottom part of the ring at about about 8 o'clock) on old shutters such as this Copal Press No. 1:

A more modern Copal shutter has both a 'B' and a 'T' setting:

The 'B' setting is for the bulb, which works the same way all the other bulb exposures do. The 'T' shutter speed is a 'time' one which uses two actuations of the release - one to open, one to close. Much like the wireless remotes today.

I have trouble finding when the 'T' was a standard setting on large format camera shutters became a standard setting (note: photos of press shutters rarely had the 'T' setting), though I've found pictures of rather old shutters that have the 'T' setting. One theory for the origins of the 'T' setting is that if you've got a hose that has some leak in it, over time the release will go on its own accord. With a 'T' setting, you can squeeze once, release, (let the pressure back into the system), squeeze again. I've got nothing to back this up other than old stories chatting with retired photographers.