Towers seem to have a habit of using Lastminute.com/Builders to get their contractors in...
Watch the "Magic Factory" programme on YouTube. On Oblivion's opening day the block paving was still being finished off with the "whacker" that gets the sane between the bricks as the guests were running into X-Sector for ERT - which started when it should be ending, i.e. half an hour late. Building materials including items such as angle-grinder discs were adorning the flowerbeds in the Oblivion queueline as planting had not been completed.
On Air's opening day 4 years later, history repeated itself. Various contractors still on-site until about 5mins before guests arrived. Equipment & materials were hastily pushed off-site to behind the "Air Gates" & also behind the Ripsaw games unit. For a good few months a huge piece of "army camouflage netting" was strung across the back of the Ripsaw games unit to the stone wall to hide the crap from guests using what was then called the Disabled Access Queue.
I totally get what you are saying, the reason for this is usually due to weather / and or unforeseen problems arising at any stage during the construction period, pushing schedules, behind schedule.
Once the schedule is behind because of weather as an example, it can be very difficult to get back on schedule. You cannot get the electronics installed if the building itself has been delayed due to weather. Or you cannot start installing track, if heavy rain has delayed the pouring of the footers they will sit on. There are many cases of this in projects, both cases, no amount of man power will resolve that issue. Even operating longer construction site hours would not resolve it, if the weather is bad, the weather is bad. Plus, due to complex structures needing to be built in certain ways, once the weather is good, it is not always simple case of hiring in more cranes to erect the structure faster.
Where and when possible and usually later in the construction once all major parts are installed, the sites will operate longer or even 24 hour operation, to try gain some of the schedule back. Alton Towers have certainly done this in the past. Such is the case when laying paths, queues, fitting out buildings, theming, foliage and all that sort of good stuff.
They were very lucky with Wickerman in there were no major things that delayed the schedule, plus they started earlier than usual. To help mitigate any problems should they arise. - sorry slightly off topic there.
Very nice to The Curse looking to be completely on schedule too, giving the global issues right now. Really excited to ride this attraction this year. Given my love of dark rides this is certainly a highlight of the last few years.