English technically doesn't have a future-tense conjugation although almost everyone, including me, calls it the future tense when "shall" and "will" are involved; however, there is no future conjugation of verbs per se. For instance:
Infinitive: to do
Simple Present Indicative: he does
Simple Past Indicative: he did
Simple Future Indicative: he will do
Emphatic Future Indicative: he shall do
The future-tense inflections haven't become lost since Old English; they've never existed. Because there has never been a verbal conjugation for the future in English, other verbs have had to be borrowed and modals have had to be formed to express this, but how? Well, in Old English, there were two verbs whose infinitives were
sculan ("have to" or "to owe")------shall
willan ("to want" or "to wish").-------will
Can you guess what words these are in Modern English? You guessed it: shall and will. These verbs were the closest things the Anglo-Saxons had to expressing the future. Obviously,
"I have to do this." = "I shall do this."
"He wants to do this." = "He will do this."
That's not really much of a future tense, is it? But that's what the Anglo-Saxons were stuck with because they had never thought to devise a future tense for their language. It wasn't their fault. The Angles, Saxons, and Jutes were Germanic tribes that invaded England at the beginning of the fifth century A.D. and, of course, since they were Germanic, that meant that they brought Germanic dialects with them replete with Germanic grammatical rules. Germanic languages do not have a future tense; Modern-day German has to do what English does to talk about the future. Talk about living in the past. Why don't Germanic languages have future tenses? I don't know; ask the Germans. Maybe the past was more interesting to them than the future because the future was uncertain; the past, though, could never be taken away. That's just my educated guess, so don't hold me to it.
So now to get back to your question: Why do "while" and "when" have to be in the present tense? Well, because there was no future tense back in Old English so that's what they had to do. "While" may have had some use of the present subjunctive as well, much like other subordinating conjunctions such as "until", "before", and "if". "When" had some use of the present subjunctive, but not consistently, and more often the subjunctive was used when the subordinating conjunction "whenever" was used rather than its namesake "when". Some examples of this, which are somewhat archaic or very formal, are:
"I shall be there waiting for James, whenever he [shall] arrive."
"Whenever you [shall] be done, Jack will pick you up."
"I shall go with you when you leave, whenever that [may] be."
NOTE: [ ] around a word means the modal used is optional and one could just use the present subjunctive form of the verb if he so wanted.
Based upon my explanation above, I think you'll find that this is the most plausible answer to the question you have posed.