Proponents of Bitcoin always mention that Bitcoin is Open Source, so if any better cryptocurrency comes along, Bitcoin can just copy those features in their open source.
Bitcoin can of course adapt to whatever its community agrees to. This isn't so much a result of the fact that it is open source, but of the fact that its use is entirely opt-in: it works because people want to use it with the properties it has. Any group at any point can decide to create their own modified network/software, possibly creating their own coin in the process. This has happened in the past, but only with very limited success.
Both Litecoin and Ripple have faster transaction speeds and more transaction capacity.
They do, but those come with significant costs too. Litecoin is fairly similar to Bitcoin, but has higher upper bounds on certain resource costs like bandwidth usage, processing time for nodes, block propagation guarantees... all metrics that influence how realistic it is to keep Bitcoin's full node network decentralized. Ripple is a completely different design that relies on wildly different security assumptions.
Note that I'm not trying to say the different choices those systems make are necessarily bad - they're just different trade-offs, and those are things to be decided by the network's ecosystem. It is highly unlikely you'll convince a sufficiently large and important subset of the ecosystem that such changes are desirable. Sure, some people will feel they are - but others won't, and unless it is clearly better in (almost) all regards, the Bitcoin ecosystem will likely prefer being conservative about such choices as it has been in the past.
So why hasn't Bitcoin just copied those features, to improve itself?
TL;DR: because not everyone considers those changes to be improvements, when taking the costs into account.
