Draft contracts are sent out early on in the transaction, along with protocol forms (F&F, title deeds/plan, Prop. Inf. Form, Lease and any other guarantees/paperwork relevant).
Then there is a gap when the solicitor has to go through all the documentation (or wait for missing documents to be provided), order searches, get the mortgage offer and raise enquiries about any issues thrown up by the paperwork. This is then batted back and forth, depending on the replies, until both sides are satisfied (this can often involve third parties to the transaction). This is when most clients are anxious to be updated, but if there is anything still outstanding, then there is not a lot to update other than "waiting for.....".
There are so many things that need to be satisfied, everything from ID to queries from the mortgage lender as well as the client not providing adequate answers and documentary evidence to queries. Clients are often just as bad at not reading the letters properly and not providing what the solicitor wants. For example, if ID needs to be certified by a professional, do get it certified by a professional listed on the form and not by your next door neighbour and if you do leave gaps on the Property Information Form, expect to get it back eventually from the other solicitor asking for you to complete it. You may sign your contract in readiness - doesn't mean your solicitor is "ready", just that it can be held on the file (i.e. one more thing ticked off).
The other biggie is that no matter how much a client stamps his foot and "demands" exchange/completion, it won't happen until both sides are legally satisfied.