Synchronicity would be the upstream and downstream being sent at the same time and regulated by the same clock source (networking equipment either contains it's own clock source to sequence it or can be connected to an external one)
What you are actually asking is why connections aren't symmetric.
The reason is cost as receive (downstream) and transmit (upstream) are separated and most applications receive more than they transmit meaning the network operator spends more on downstream bandwidth than upstream as this means they can use this as a shared resoruce and distribute the cost among the customers because it does not require permanently dedicated bandwidth though the backhaul from a street cabinet or exchange should have capacity to cope with all customers using their connection to the max.
An organiation with a lot of employees or that have employees connecting in via VPN or that hosts it's servers that customers connect to will typically have a dedicated symmetric fibre circuit but as this requires dedicated resource a telco will charge more for it (they typically come with various service level agreements that impose penalties on the telco if faults aren't fixed within so many hours as well as round the clock support)
Mobile networks are harder to plan because the base is well, mobile so based on what I have said above it's unlikely there'll be a true symmetric service.
Happy to explain more of the technicalities for anyone interested but bit too shattered tonight.
