They don't, it's a question of DSL profiles. Commonly used consumer DSL profiles have a 8:1 to ~2:1 ratio because that seems to match well with consumer usage. Business DSL generally is symmetric.
1000 MBit/s downstream, 35 MBit/s upstream is an 30:1 ratio, far more extreme than any DSL profile.
DSL, at least ADSL commonly found in homes, is designed that way as a good way to split the frequency available on the line to match what people need. It also shares a good portion of the frequency with POTS telephone service. There are SDSL (symmetric DSL) standards which provide fully symmetric bandwidth on the circuit. Not sure how popular it is these days but when I supported it at an ISP about 10 years ago, it was a cheap alternative to T1-class circuits.
They don't technically. I remember there being a provider here in Germany where you could switch your connections profile around. (afaik something like 6:1, 1:1, 1:6) This was way back when speeds really were an issue, so freelance web developers etc loved them. Haven't seen anything like that for a while though, at least in the home offerings. (but I also would expect demand to be quite low for it)
DSL is dedicated all the way back to the central office, so it doesn't have the same problem as cable has. That said, what others posted is correct, home users download far more than they upload, so it makes sense from a marketing perspective to give far more downstream bandwidth than upstream. Home users tend to appreciate having a high download speed much more than having equal, but much lower upload and download speeds.
Other interesting info: ADSL is dreadfully slow by today's standards, but VDSL can actually hit some decent speeds. The big problem with both is attenuation of copper wire is fairly significant over the distance the lines generally travel, much more so with VDSL, due to it's higher frequency range. That's why companies like AT&T, with their U-Verse brand, bring fiber to the neighborhood, to lop of a significant distance of copper. They can increase the distance they service by making use of two lines paired together. AT&T takes a portion of legacy ADSL service now and puts VoIP over it, with the VDSL service having more than half it's capacity generally devoted to IPTV bandwidth. One single good condition line could generally carry 118 Mbps, but they would split that up for TV, leaving only 45 Mbps. Now they are mostly pushing FTTH/FTTP though, leaving no option for POTS/copper. That is all good though, because most the copper run is such garbage that it absolutely sucks being a technician. You would be gigged for the line runs getting water in them and making service drop out, even if everything tested great when you installed, or even if you turned a ticket in the line techs might kick it back and say everything is fine.
I'd also like to mention something that was tried in Serbia (but ultimately failed, though in a good way):
Our national telecom operator, Telekom Srbija, had a monopoly on internet access in most areas due to the fact they own all the phone lines. In cooperation with Huawei and the Chinese and Serbian governments, they made a plan to kill POTS service per line and use ALL of the bandwidth for VDSL, as well as building DSLAMs on street level, to provide very high speeds (100/10 Mbps was the max they achieved before stopping) with short copper lengths. Phones would work over VoIP, so all new modems were VDSL compliant with IAD (integrated access device) "certification" (a SIP client and 2 POTS ports).
In the end the Chinese lost interest and it was all forgotten and covered with ash. The new idea, which is why I say it failed in a good way, is to skip the street DSLAM buildout and just do FTTH. And they did, though not with symmetric gigabit speeds, because reasons (bla bla customer demand, costs, etc..) The highest speed is 1000/400 Mbps for $120/mo. (which is super expensive compared to what was promised by the Chinese and also compared to general purchasing power in the country).
Well.... yes, you do, and some people in the US have 4/0.5 Mbps for the same price. It really depends.
For example, in Niš, which is the most developed city when considering internet infrastructure and the number of ISPs, you could have 600/400 Mbps for $10/mo with ~300 TV channels on unlimited* TVs. You'd even get a [VoIP] landline.
* all the TVs in your home, basically up to 5 for 99% of users
Then Telekom Srbija got the Chinese money and bought 10 largest ISPs other than them and SBB (the other side of the duopoly), and now there's 200/20 Mbps on FTTH for $30/mo on up to 3 TVs with 260 channels and a landline that has call routing problems. Users which sign new contracts are downgraded and pay more, those that don't sign new contracts suffer with TV and Internet dying, and are told to "sign or sue us".
Ironically, I chose the second option, and won (https://news.ycombinator.com/item?id=21733553) but the thing I "won" is free contract termination and being repaid fees that were charged but are not mentioned or required under the TOS and Contract at the date of signing (given I have not signed anything else giving permission or agreeing to anything new, and they didn't officially force new contracts on people so that people can't cancel to run away)...
