The problem with that is the manufacturer doesn't know what type of mic the end-user will want to actually use. I would rather not have to crack open the radio every time I switch mics to adjust the mic level if the mic doesn't have the control to do so itself.
Most after purchase add on mikes come with variable gain and sometimes tone too,if the radio is set up well internally just about any mike will work well on it,if the internal mike gain is set too high then some after purchase mikes will over drive the radio.
realistically there isn't that many add on mikes that will actually improve your audio,most just add background noise or other undesireable traits.The one mike that did seem worthwhile the astatic 575-m6 has been discontinued.
every time i hear bad audio,no matter what mike is used its because the owner decided that amc or fm deviation needed reset or increased to ludicrous levels or defeated completely and not because their external mike gain was too high.
The only ways i can see mike gain having any worthwhile value is reducing ssb drive or if your the type of person who likes hacking out limiters,which in itself is self destructive to good audio.Mike gain in itself is misnamed anyway,its a mike attenuator,not a gain control,the max gain is set internally,so its basically just a knob for knob twiddlers to play with,it has no real useful purpose if the radio is adjusted properly internally.
as for echo,in 32 years of using radios,mostly on ssb,i've never heard a single echo that improved audio,it is a completely worthless accessory for people who have seriously deformed hearing,a processor is the exact opposite,while it does detract from audio quality the trade off and extra oomph that a small amount of processing gives in poor conditions,can be the difference in making or breaking a contact.