Because your penis - foreskin and all - is there for sex.
Masturbating with the foreskin retracted and using lubricant is an option if you have an intact foreskin, it isn't the "natural" way to do it because the foreskin provides a very effective sliding action within its inner and outer layers.
It certainly is the case that circumcised men are more restricted in their options, either managing to draw enough skin up and over the glans, and adopting a technique which maintains this as they proceed, or resorting to "artificial" lubrication (which makes it a rather messy business).
Pre-ejaculatory secretion ("pre-come") - which is absolutely not semen and therefore contains no meaningful amount of sperm - is produced by the para-urethral and Cowper's glands which line the inside of the urethra all along the part that runs through the penis and just inside the pelvis. Interestingly enough, the production of this fluid is not necessarily coincident with erection, but appears to be separately provoked more by the perception of sexual proximity to a female partner, which is of course, usually missing when you masturbate.
Unless perhaps (and I always recommend against this,) you are viewing pornography, you are actually not so likely to produce significant amounts of this, and it is not surprising that you need to use something else if you choose a form of masturbation requiring lubrication. Saliva tends to "wear out" very rapidly, which is why as I always explain, "blow jobs" tend to be less satisfactory than you imagine. (On the other hand, the young lady indulging in a very leisurely oral session, not focused on making you ejaculate, may be quite delighted to enjoy the regular little drips of the quite sweet "pre-come" which she tastes as she proceeds.)
Fitting three or more fingers inside the foreskin is not only doable, but is the proper way to perform the stretching as it relieves you of the need to use your muscles to maintain the tension; just "wedge" in as many fingers as will fit and let them sit there.
If you are observing an imbalance of the amount of skin above and below your glans, it confirms that you do need to perform the frænulum stretches at least as much as stretching the opening.
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