Believe it or not other posters have answered your question very succinctly, but due to your frustration at not understanding all of it you may have not yet procesed what they have written yet.
IN some ways im glad you are frustrated with this because it shows you really want to understand music. You do need to relax a little tho, because the whole system of western music is built on a bunch of concepts that take a long time to understand and a lot of persistence. If it doesn't make sense initially, you may need to start at the beginning.
Im going to attempt an explanation, because as someone with a degree in music theory and a full time jazz musician and conservatorium guitar teacher i think understand your frustrations with the nomenculature of dom 7 chords.
However, I am afraid that if i write something that you will throw your hands up and have a tantrum like you have with the other posters. THe first thing you need to undersdtand is the concept of intervals. If you really want to get to the bottom of this you will need to do some research on your own and im happy to reccommend some avenues to explore. If you are, then send me a PM.
However you seem to want a quick and dirty answer to your problem: so here is an attempt quick and dirty explanation of why 7th chords are called that. There is a lot more to understand about chord function, but this might at least address some of your frustrations.
1. Chords are constructed from triads.
2. Triads are created by using the 1st, 3rd, and 5th notes of any scale. There are many types of scales.
3. There a only 4 types of triads: major (1,3,5), minor (1,b3,5), augmented (1,3,#5) and diminished (1,b3,b5)
4. Chords constructed using additional voices to the triads are collectively known as 7th chords because the include both the triad (1,3,5) and the 7th degree of the scale.
5. A MAJOR 7th chord is based on the major scale and includes therefore 1,3,5 and 7...in the key of c for example, that would be the notes C,E,G and B.
A MINOR 7th chord contains 1,b3,5 and b7
Dominant 7th chords (or just 7th chords for short) are created using the 1,3,5 and flat7.
There are even diminished 7th chords that contain, 1,b3,b5 and a double flattened 7!
...now here is i think why you are getting frustrated....you want to know why the 7th chord (ie. the one with the flattened 7th) is called the 7th chord when really it includes a flat seven. Cos it seems more logical to call the Maj 7 a 7 and a major triad with a flat 7 a major flat 7 chord Right?
Heres why......its actually to save time believe it or not.
Imagine an E chord (1,3,5) with a flat 7 (currently recognized as E7).....would you call it Eb7, or even e b7? or perhaps e maj b7? Bad idea...because that seems more like its an Eb chord with the 7th on top right? So to save confusion when analyzing chord structures of songs, it had become the norm to call any chord with a flattened 7, just a 7 chord.
Heres some examples where just calling a major triad with a flat 7 just a 7 chord is easier:
standard and simple: g7,f#7,gb7
difficult and confusing: g b7 (or is that a gb7?), f#b7 (is that f sharp or f flat 7?), gb7 (is that G with a flat 7, or Gb with a seven?)
For now, just accept that major chords with a flat seven are called 7th and major chords with a regular 7th are called maj7. IN the meantime, if this confuses you still, then you need to start at the beginning and learn about intervals and keys and all that stuff. Im happy to help start at the start if you want to send a PM.
