Why are the rifling grooves so deep in modern traditional rifles?
It is the current fad of customer demand.
Aren’t the deeper grooves less likely to be wiped clean after a shot when he next patched ball is loaded?
Yes- maybe
And isn’t it harder to get any kind of a gas seal with really deep grooves?
Definitely.
In a hypothetical barrel, say 45 cal, anything over 0.010" is deeper than provides best result, IMHO. I have used 45 cal cartridge barrels for MLs. The depth of groove is only 0.004" and they shoot great. They are easier to get to shoot well than deep grooves. TC used button rifle barrels and they are loved by most. They are about 0.006" deep. An advantage of shallow rifling is the ability to shoot Maxi and REAL bullets.
The optimum depth of riffling is caliber dependent.
I bought a deep groove round bottom 45 barrel for my club shoot rifle to see for myself. It was really awful. It never shot well enough to suit me despite dozens of load combinations. It appeared to be leaking gas in the middle of the grooves with any loadable combination. I scrapped it, and replaced it with a regular GM barrel, that barrel is very accurate. IMHO round bottom barrels have no advantage at all.
Another consideration is wide vs narrow grooves. IF the grooves are narrow and deep you will probably have a lemon of a barrel. I look for a 40/60 or better a 30/70 ratio. Narrow lands and wide grooves are better up to a point. IF the lands are super narrow then they are subject to early loading wear.
These days you are lucky to find any barrels in stock. Being picky about these details means having a custom barrel made and that takes time.