FWIW, a decent sized gas tankless WH is likely in the range of 200K BTU...converting that to watts, that's about 58.6Kw. At 240vac input, that's 244A. Since that load is outrageous, and most people could never support one that big, the things tend to be much smaller, and as a result, have a MUCH smaller hot water temperature rise, OR a much smaller maximum volume (often, both). IMHO, an electric tankless is a joke except for maybe a single point of use (most likely a sink, or a really low-flow shower).
An electric powered TANK WH works because it can take its merry time to reheat the water (assuming you get one big enough to meet your needs - doesn't work well at all for continuous or commercial use where you need continuous hot water).
The two arguments for tankless are: lower standby losses (the newer tanks are often quite good in this respect), and longevity (most people never service their WH, and only replace it when it leaks - try that with a tankless that costs lots more to buy and install and you'll have a useless piece of junk - throw in that there often aren't that many qualified service people for tankless, and the costs often just don't compute.
A third argument is that they are often smaller, and people want or need the room.